Fix some bugs introduced in c57baae4775f4e665b7e158582234ac558a2bf7b preventing the 'mime' routing function from working with request variables, and actually return the found mimetype

The uwsgi_route_var_mime function had two errors preventing it from (afaics ever) working:
 1. Redefine the 'ret' variable inside an if block. This meant that if the uwsgi_get_mime_type function ever actually found a matching mimetype, the length of the matching value would be corectly set, but the function returns NULL (the value of the function-level ret variable)
 2. The function was looking up the request variable correctly, BUT then passing in the raw config string to uwsgi_get_mime_type NOT the resolved varible value, making the lookup just a waste of cycles.
Merge pull request #585 from stestagg/master

Fix uwsgi_route_var_mime