implemented method : updateTestCaseCustomFieldDesignValue

Hi,

I needed the updateTestCaseCustomFieldDesignValue from the XMLRPC service, so I just implemented it in the API.
Two versions of it, which do not have the same args.

I could'nt implement Unit Test, since the XMLRPC does not return any feedback for this method, so we cannot check if we actually changed the value of the customField.

However, if you have any idea of how to do that, or if you have any suggestion about this pull request, I would be glad to know them.

Best regards,

Thibault Lasnier