Fix Output Order in RSpec Error Messages

Prior to this failed assertions would indicated that the *expected* XML was the subject of the assertion. 

I was using  [this commit](https://github.com/sshaw/test_xml/commit/8dd4147cefba441bcf3b75cbd9e7608f83672392) for my testing, which adds a diff to the failed assertion message, something I think is a must, particularly for large XML docs. I made this change a while ago (just finally forked your repo and committed) and, if I recall, I think the diffing is naive and can lead to a confusing diff. At least it can be used as point of discussion.