The Merge methods API is now official (#562)

* The Merge methods API is now official

see: https://developer.github.com/changes/2017-04-07-end-merge-methods-api-preview/
this removes the apiVersion (that wasn't being used anyway..)
also adds support for rebase mergeMethod per https://developer.github.com/v3/pulls/#merge-a-pull-request-merge-button

Since this previously looked for the value to be bool, this is a breaking change

* retain weird bool support for this value

* let github throw errors instead

* add back in $apiVersion to make it easier to handle preview versions in the future

there are no preview versions now so checking against an empty array
also had to fix the acceptHeaderValue to correctly use the variable

* throw invalidargumentexception when trying to use invalid merge method

* add missing use

Author: bobeagan

lib/Github/Api/PullRequest.php

@@ -5,6 +5,7 @@
 use Github\Api\PullRequest\Comments;
 use Github\Api\PullRequest\Review;
 use Github\Api\PullRequest\ReviewRequest;
+use Github\Exception\InvalidArgumentException;
 use Github\Exception\MissingArgumentException;
 
 /**
@@ -29,7 +30,7 @@ class PullRequest extends AbstractApi
      */
     public function configure($bodyType = null, $apiVersion = null)
     {
-        if (!in_array($apiVersion, array('polaris-preview'))) {
+        if (!in_array($apiVersion, array())) {
             $apiVersion = $this->client->getApiVersion();
         }
 
@@ -41,7 +42,7 @@ public function configure($bodyType = null, $apiVersion = null)
             $bodyType .= '+json';
         }
 
-        $this->acceptHeaderValue = sprintf('application/vnd.github.%s.%s', $this->client->getApiVersion(), $bodyType);
+        $this->acceptHeaderValue = sprintf('application/vnd.github.%s.%s', $apiVersion, $bodyType);
 
         return $this;
     }
@@ -182,6 +183,10 @@ public function merge($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,