Changed the key for the MessageBodyReader cache.

The reason for this change is that there is a cache of MessageBodyReaders that grows until memory runs out.
This happens when getting lots of multipart messages (with unique boundaries). There is a cache (MessageBodyFactory.mbrLookupCache) that eventually will fill up the available memory because boundaries make the keys unique and new entries are added to the cache for each request.

This change will create a new key for the cache that doesn't contain the additional parameters and won't
make for example the multipart/mixed media type uniqe just because the boundaries are uniqe.