Add have_correct_syntax spec and fix have_correct_syntax Haml support

I've added the support for Haml in `have_correct_syntax` and then completely forgot about it and didn't even see if it works, sorry!

This pull request fixes the Haml support in `have_correct_syntax` and adds necessary tests. Now the new matchers have full coverage so there shouldn't be any more pull requests in this area for you to merge.

If you're still willing to give me the commiting privilleges, I'll gladly accept them after all ;)