Adds test cases for Accept, including best_match

One test is an expected failure. This is because the
current implementation does not correctly implement
specificity. I tried decorating this with
@unittest.expectedFailure, but that didn't work for
python 2.6 so I've now commented out that test instead.

The problem is if the accept header on the request is:
`Accept: x;q=0,y;q=0.5,*;q=1`
Then `x` should never be the best match because it has
been specifically declared as q=0. However, in the current
implementation, werkzeug will see *;q=1 first and decide
that everything matches the wildcard. This is not correct
behaviour according to RFC2616, as an `x` should match the
specific entry for `x` rather than the wildcard entry `*`,
but more substantial changes would be needed to fix this.
Merge branch 'fix/accept-best-matches-quality-zero' of https://github.com/wtsi-hgi/werkzeug into fix/accept-best-matches-quality-zero