Fix encoding errors when hashing XML

We occasionally get UnicodeEncodeErrors from these two places,
because the XML is a Unicode string, and is implicitly encoded to ascii.
Change a test to get at some Unicode issues.
Fix weird formatting.
Make the repr right.
Write a test for the XML unicode conversion fix.
Add to the test, a check that the XML file truly has a non-ASCII character in it.
Use repr instead of str for getting context data via the system render function, so it works better for non-ASCII data.
Test the Unicode handling during construction of an ErrorModule directly.
In these tests, 'rendered_html' is neither rendered, nor HTML.  Name it context_repr, since that's what it is.
Merge pull request #94 from edx/ned/fix-silly-xml-encoding-errors

Fix encoding errors when hashing XML