Don't overwrite TestCase.server_url with None.

The passed-in config could have server_url=None is it's not specified in
the config or command-line options.  This should not overwrite an explicit
value of server_url from the class.
Merge pull request #34 from mozilla-services/rfk/dont-overwrite-serverurl-with-none

Don't overwrite TestCase.server_url with None.