Add and detect AbuseRateLimitError.

It's similar to RateLimitError, but it's a different type of rate limit
error. It is documented at:

https://developer.github.com/v3/#abuse-rate-limits

Parse and include the Retry-After header value, if present. It is
documented at:

https://developer.github.com/guides/best-practices-for-integrators/#dealing-with-abuse-rate-limits

According to GitHub support, its type is an integer value representing
seconds.

Helps #431.

github/github.go

@@ -494,6 +494,24 @@ func (r *RateLimitError) Error() string {
 		r.Response.StatusCode, r.Message, r.Rate.Reset.Time.Sub(time.Now()))
 }
 
+// AbuseRateLimitError occurs when GitHub returns 403 Forbidden response with the
+// "documentation_url" field value equal to "https://developer.github.com/v3#abuse-rate-limits".
+type AbuseRateLimitError struct {
+	Response *http.Response // HTTP response that caused this error
+	Message  string         `json:"message"` // error message
+
+	// RetryAfter is provided with some abuse rate limit errors. If present,
+	// it is the amount of time that the client should wait before retrying.
+	// Otherwise, the client should try again later (after an unspecified amount of time).
+	RetryAfter *time.Duration
+}
+
+func (r *AbuseRateLimitError) Error() string {
+	return fmt.Sprintf("%v %v: %d %v",
+		r.Response.Request.Method, sanitizeURL(r.Response.Request.URL),
+		r.Response.StatusCode, r.Message)
+}
+
 // sanitizeURL redacts the client_secret parameter from the URL which may be
 // exposed to the user, specifically in the ErrorResponse error message.
 func sanitizeURL(uri *url.URL) *url.URL {
@@ -564,6 +582,20 @@ func CheckResponse(r *http.Response) error {
 			Response: errorResponse.Response,
 			Message:  errorResponse.Message,
 		}
+	case r.StatusCode == http.StatusForbidden && errorResponse.DocumentationURL == "https://developer.github.com/v3#abuse-rate-limits":
+		abuseRateLimitError := &AbuseRateLimitError{
+			Response: errorResponse.Response,
+			Message:  errorResponse.Message,
+		}
+		if v := r.Header["Retry-After"]; len(v) > 0 {
+			// According to GitHub support, the "Retry-After" header value will be
+			// an integer which represents the number of seconds that one should
+			// wait before resuming making requests.
+			retryAfterSeconds, _ := strconv.ParseInt(v[0], 10, 64) // Error handling is noop.
+			retryAfter := time.Duration(retryAfterSeconds) * time.Second
+			abuseRateLimitError.RetryAfter = &retryAfter
+		}
+		return abuseRateLimitError
 	default:
 		return errorResponse
 	}

github/github_test.go

@@ -538,6 +538,71 @@ func TestDo_rateLimit_noNetworkCall(t *testing.T) {
 	}
 }
 
+// Ensure *AbuseRateLimitError is returned when the response indicates that
+// the client has triggered an abuse detection mechanism.
+func TestDo_rateLimit_abuseRateLimitError(t *testing.T) {
+	setup()
+	defer teardown()
+
+	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json; charset=utf-8")
+		w.WriteHeader(http.StatusForbidden)
+		// When the abuse rate limit error is of the "temporarily blocked from content creation" type,
+		// there is no "Retry-After" header.
+		fmt.Fprintln(w, `{
+   "message": "You have triggered an abuse detection mechanism and have been temporarily blocked from content creation. Please retry your request again later.",
+   "documentation_url": "https://developer.github.com/v3#abuse-rate-limits"
+}`)
+	})
+
+	req, _ := client.NewRequest("GET", "/", nil)
+	_, err := client.Do(req, nil)
+
+	if err == nil {
+		t.Error("Expected error to be returned.")
+	}
+	abuseRateLimitErr, ok := err.(*AbuseRateLimitError)
+	if !ok {
+		t.Fatalf("Expected a *AbuseRateLimitError error; got %#v.", err)
+	}
+	if got, want := abuseRateLimitErr.RetryAfter, (*time.Duration)(nil); got != want {
+		t.Errorf("abuseRateLimitErr RetryAfter = %v, want %v", got, want)
+	}
+}
+
+// Ensure *AbuseRateLimitError.RetryAfter is parsed correctly.
+func TestDo_rateLimit_abuseRateLimitError_retryAfter(t *testing.T) {
+	setup()
+	defer teardown()
+
+	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json; charset=utf-8")
+		w.Header().Set("Retry-After", "123") // Retry after value of 123 seconds.
+		w.WriteHeader(http.StatusForbidden)
+		fmt.Fprintln(w, `{
+   "message": "You have triggered an abuse detection mechanism ...",
+   "documentation_url": "https://developer.github.com/v3#abuse-rate-limits"
+}`)
+	})
+
+	req, _ := client.NewRequest("GET", "/", nil)
+	_, err := client.Do(req, nil)
+
+	if err == nil {
+		t.Error("Expected error to be returned.")
+	}
+	abuseRateLimitErr, ok := err.(*AbuseRateLimitError)
+	if !ok {
+		t.Fatalf("Expected a *AbuseRateLimitError error; got %#v.", err)
+	}
+	if abuseRateLimitErr.RetryAfter == nil {
+		t.Fatalf("abuseRateLimitErr RetryAfter is nil, expected not-nil")
+	}
+	if got, want := *abuseRateLimitErr.RetryAfter, 123*time.Second; got != want {
+		t.Errorf("abuseRateLimitErr RetryAfter = %v, want %v", got, want)
+	}
+}
+
 func TestDo_noContent(t *testing.T) {
 	setup()
 	defer teardown()