Accept header parsing fixes. Now return first acceptable match rather than last. If not acceptable match in accept, 406 error returns list of acceptable types as text string. application/xml is listed in acceptable types only if dicttoxml is installed. Handle q > 1.0 by demoting q factor to 0.0001 making it unusable. Test cases for all this code. XML is commented out as we don't install dicttoxml.py.

[run]

# ignore the dist utils. They are not part of the code base
# we are going to test. They are used by maintainers only.
omit = roundup/dist/*