Accept valid Accept headers in swob.

"Accept: application/xml; charset=UTF-8" is totally valid, and has an
implicit q (quality) value of 1.0, just the same as "Accept: text/xml"
does.

Also, you can say things like:
Accept: text/xml; charset=UTF-8; q=0.9; anglebrackets="are awesome"
with as many arbitrary extensions as you want.

See RFC 2616 sections 14.1 Accept and 2.2 Basic Rules for details.

Fixes bug 1202453.

Change-Id: I18e6d0ee3fd6f9d889275ee8335e711c729b7171
Merge "Accept valid Accept headers in swob."