use *[]string for IssueRequest.Labels

Unfortunately this is a breaking change, and having a pointer to a slice
is not very common, but is needed in this case to allow clearing an
issue's labels.

Fixes #181

github/issues.go

@@ -49,12 +49,12 @@ func (i Issue) String() string {
 // It is separate from Issue above because otherwise Labels
 // and Assignee fail to serialize to the correct JSON.
 type IssueRequest struct {
-	Title     *string  `json:"title,omitempty"`
-	Body      *string  `json:"body,omitempty"`
-	Labels    []string `json:"labels,omitempty"`
-	Assignee  *string  `json:"assignee,omitempty"`
-	State     *string  `json:"state,omitempty"`
-	Milestone *int     `json:"milestone,omitempty"`
+	Title     *string   `json:"title,omitempty"`
+	Body      *string   `json:"body,omitempty"`
+	Labels    *[]string `json:"labels,omitempty"`
+	Assignee  *string   `json:"assignee,omitempty"`
+	State     *string   `json:"state,omitempty"`
+	Milestone *int      `json:"milestone,omitempty"`
 }
 
 // IssueListOptions specifies the optional parameters to the IssuesService.List