MailCore/mailcore2 — pull request #1014 by KieranMcGrady (2014-12-22 10:23:20+00:00)
Title: Added assertion to check folder isn't NULL

This PR asserts that the folder string isn't NULL before returning the operation. It's based on [this](http://stackoverflow.com/questions/27567166/mailcore-plaintextbodyrenderingoperationwithmessage-returns-nil/27567463) StackOverflow post.