Fix encoding issues in PDFTemplateResponse with python 2.x

Issue #13 fixed the problem for python 3.x but not for python 2.x. I fixed the problem and also updated the tests to test PDFTemplateResponse and PDFTemplateView (the old tests passed but in fact, they were not testing unicode content at all).

This also solves issue #51. Let me now if I missed something.
Oh I see it now - too many things to do at once :)