Fix mimetype routing priority.

When an `urlSuffix` e.g. .json is specifed by the user, this should
override any supplied `Accept` http headers. The previous method
of only iterating once and calling `canFormat()` for each mime type
made that impossible. We now have two methods, `matchFileExtension()`
and `matchAcceptTypes()` that we can call separately, allowing us
to iterate first looking for url suffixes and if none match, falling
back to accept headers.
Merge pull request #33 from codemix/fix-mimetype-priority

Fix mimetype routing priority.