Fix misspelling in GistCommit.CommittedAt field. (#564)

It's technically a breaking change in that anyone who was referencing
the old field will now have code that won't compile, since the name
changed. However, since the JSON tag was wrong too, it means that the
field was never actually getting populated. So in that sense, it's not
actually a break in behavior.

The fixed version now matches the GitHub API and will have its value
correctly populated.

Author: shawnps

github/gists.go

@@ -59,7 +59,7 @@ type GistCommit struct {
 	Version      *string      `json:"version,omitempty"`
 	User         *User        `json:"user,omitempty"`
 	ChangeStatus *CommitStats `json:"change_status,omitempty"`
-	CommitedAt   *Timestamp   `json:"commited_at,omitempty"`
+	CommittedAt  *Timestamp   `json:"committed_at,omitempty"`
 }
 
 func (gc GistCommit) String() string {

github/gists_test.go

@@ -302,7 +302,7 @@ func TestGistsService_ListCommits(t *testing.T) {
 		        "additions": 180,
 		        "total": 180
 		      },
-		      "commited_at": "2010-01-01T00:00:00Z"
+		      "committed_at": "2010-01-01T00:00:00Z"
 		    }
 		  ]
 		`)
@@ -314,10 +314,10 @@ func TestGistsService_ListCommits(t *testing.T) {
 	}
 
 	want := []*GistCommit{{
-		URL:        String("https://api.github.com/gists/1/1"),
-		Version:    String("1"),
-		User:       &User{ID: Int(1)},
-		CommitedAt: &Timestamp{time.Date(2010, 1, 1, 00, 00, 00, 0, time.UTC)},
+		URL:         String("https://api.github.com/gists/1/1"),
+		Version:     String("1"),
+		User:        &User{ID: Int(1)},
+		CommittedAt: &Timestamp{time.Date(2010, 1, 1, 00, 00, 00, 0, time.UTC)},
 		ChangeStatus: &CommitStats{
 			Additions: Int(180),
 			Deletions: Int(0),