add headBranch field on CheckSuite (#117)

Co-authored-by: Daniel Wallman <[email protected]>

Author: Wallman

src/main/java/com/spotify/github/v3/checks/CheckSuite.java

@@ -39,4 +39,6 @@ public interface CheckSuite {
   Integer id();
 
   Optional<App> app();
+
+  Optional<String> headBranch();
 }

src/test/java/com/spotify/github/v3/activity/events/CheckRunEventTest.java

@@ -44,6 +44,7 @@ public void testDeserialization() throws IOException {
         assertThat(checkRunEvent.action(), is("created"));
         assertThat(checkRunEvent.checkRun().name(), is("Octocoders-linter"));
         assertThat(checkRunEvent.repository().name(), is("Hello-World"));
+        assertThat(checkRunEvent.checkRun().checkSuite().get().headBranch().get(), is("changes"));
     }
 
 }