[Regression] Allow Basic Auth by a username with no password

Gemfury currently relies on Bundler's support for username-only authentication with a source such as:

    https://secret-token@gem.fury.io/username/

Bundler 1.3 supported this authentication method, however the latest `1.4.0.pre1` release is causing `HTTPForbidden` errors for our customers because the authentication token is not passed to our server.

This PR is to continue supporting username-only basic authentication.