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.

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   Design (original) <docs/design>
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