Handle parse_accept_lang_header error

Handle bug described in https://code.djangoproject.com/ticket/21078
The fix has been merged into the Django master branch, but won't be in
a Django release for some time.

Also added test coverage for funfactory.urlresolvers.Prefix class.
Merge pull request #53 from jgmize/handle-parse_accept_lang_header-error

Handle parse_accept_lang_header error