added username to logon_with_openid to script CEDA access more easily

[cojacoo]

Hi, Thank you so much for this package. I ran quite sometimes into a minor issue with the logon function when CEDA actually required a username but self._get_logon_details(openid) would not provide them. I suggest a very small change to allow for explicit username provision to the logon_with_openid function. If left empty, everything will work as before. If provided, the username is propagated to the logon function. This fix solves the issue for me. All the best. Conrad

[agstephens]

Hi @cojacoo , the change looks fine but I would rather we had an extra unit test or two to support it. Could you add the test(s) and include them in the Pull Request? Many thanks