bug #881 fix: use new media type for branch protections (iBotPeaches)

This PR was squashed before being merged into the 2.15.x-dev branch.

Discussion
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Not sure when this changed, but `loki` is not the preview anymore. It is `luke-cage`. Without this fix you won't get back branch protection values. (like required amount of reviewers).

https://developer.github.com/v3/repos/branches/#get-branch-protection

If you need to support commit signature crud, that has a different media type under Protections, but I don't see that supported at this time.

Commits
-------

1b767b3 fix: use new media type for branch protections
3e821c4 fix: set preview header for endpoints that require it only
ab9241e Revert "fix: use new media type for branch protections"

Author: iBotPeaches

lib/Github/Api/Repository/Protection.php

@@ -34,6 +34,9 @@ public function configure()
      */
     public function show($username, $repository, $branch)
     {
+        // Preview endpoint
+        $this->acceptHeaderValue = 'application/vnd.github.luke-cage-preview+json';
+
         return $this->get('/repos/'.rawurlencode($username).'/'.rawurlencode($repository).'/branches/'.rawurlencode($branch).'/protection');
     }
 
@@ -51,6 +54,9 @@ public function show($username, $repository, $branch)
      */
     public function update($username, $repository, $branch, array $params = [])
     {
+        // Preview endpoint
+        $this->acceptHeaderValue = 'application/vnd.github.luke-cage-preview+json';
+
         return $this->put('/repos/'.rawurlencode($username).'/'.rawurlencode($repository).'/branches/'.rawurlencode($branch).'/protection', $params);
     }