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

See http://stackoverflow.com/a/4832418/27206 for some context on
text/xml vs. application/xml.

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.
Merge pull request #266 from jagregory/master

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