Use MIMEAccept instead of Accept

Accept does not take into account type/subtype, and thus could
potentially return the wrong result to a client if they send a funky
Accept header.

Closes #240

webob/exc.py

@@ -170,7 +170,7 @@
 import re
 import sys
 
-from webob.acceptparse import Accept
+from webob.acceptparse import MIMEAccept
 from webob.compat import (
     class_types,
     text_,
@@ -327,9 +327,9 @@ def generate_response(self, environ, start_response):
             del self.content_length
         headerlist = list(self.headerlist)
         accept_value = environ.get('HTTP_ACCEPT', '')
-        accept = Accept(accept_value)
         match = accept.best_match(['application/json', 'text/html',
                                    'text/plain'], default_match='text/plain')
+        accept = MIMEAccept(accept_value)
         if match == 'text/html':
             content_type = 'text/html'
             body = self.html_body(environ)