prevent partial matches

see https://github.com/willdurand/Negotiation/commit/29ed0cab65ce352c6ff1cde0b053c91c8a6b2bb6#commitcomment-4104094

f.e. this also makes it impossible to register a new format ``jsonp`` as ``application/javascript+jsonp`` because it will already be matched to ``js`` (semi related to this is the issue that currently there is no way to force the new format to be placed in a specific location in the existing formats array).