Fix issue where milestones API did not accept "all".

Fryguy · Fryguy · commit 809002e0f765 · 2014-08-26T18:01:42.000-04:00
See GitHub API reference https://developer.github.com/v3/issues/milestones/#parameters It is already documented that it is supported in Milestones#list https://github.com/peter-murach/github/blob/c71f4fb8f162e0d6bc12bc8ef0aa190f953ad65e/lib/github_api/client/issues/milestones.rb#L23
diff --git a/lib/github_api/client/issues/milestones.rb b/lib/github_api/client/issues/milestones.rb
@@ -4,7 +4,7 @@ module Github
   class Client::Issues::Milestones < API
 
     VALID_MILESTONE_OPTIONS = {
-     'state' => %w[ open closed ],
+     'state' => %w[ open closed all ],
      'sort'  => %w[ due_date completeness ],
      'direction' => %w[ desc asc ]
     }.freeze # :nodoc:
