Include application/xml in the content types that trigger the xml parser

I noticed responses with a content-type of `application/xml` weren't being treated the same as `text/xml` responses. I've modified pluggable_parsers to treat them the same.

I had to make some minor changes to pluggable_parsers to allow multiple
content types per parser; this is only really necessary to support the
xml= style methods.

See http://stackoverflow.com/a/4832418/27206 for some context on `text/xml` vs. `application/xml`.

Cheers