don't require Preference-Applied response header

The Preference-Applied header isn't required by LDP or RDF 7240. Since
we can tell if the client hint was honored from the content, don't fail
servers that don't use the header. If the header is present, however,
make sure it contains return=representation.

Fixes #155
Merge pull request #163 from spadgett/master

don't require Preference-Applied response header