Encode markdown html text to utf-8 before passing into minidom.

minidom doesn't seem to like unicode objects (and tries to decode them as if
they were ascii). The trivial fix is to encode to utf-8 before calling
parseString.

Testing done:
Looked at /r/2568/ from r.rb.o, which previously caused a traceback. Saw that
both the description and the change description field correctly rendered the
unicode text.