fix unit tests for go1.7 release

An upcoming change to Go 1.7 allows the json package to be able to encode maps
with the integer keys. Thus, this is no longer a good way to generate intentional
JSON failures.

Reference:
	f05c3aa24d

github/github_test.go

@@ -198,7 +198,7 @@ func TestNewRequest_invalidJSON(t *testing.T) {
 	c := NewClient(nil)
 
 	type T struct {
-		A map[int]interface{}
+		A map[interface{}]interface{}
 	}
 	_, err := c.NewRequest("GET", "/", &T{})