throw invalidargumentexception when trying to use invalid merge method

Author: bobeagan

lib/Github/Api/PullRequest.php

@@ -163,6 +163,10 @@ public function merge($username, $repository, $id, $message, $sha, $mergeMethod
             $mergeMethod = $mergeMethod ? 'squash' : 'merge';
         }
 
+        if (!in_array($mergeMethod, array('merge', 'squash', 'rebase'), true)) {
+            throw new InvalidArgumentException(sprintf('"$mergeMethod" must be one of ["merge", "squash", "rebase"] ("%s" given).', $mergeMethod));
+        }
+
         $params = array(
             'commit_message' => $message,
             'sha' => $sha,