Pull mirroring doesn't allow credentials to be unset

Summary

Introduced in the resolution to #98 (closed) . Unsetting the username, password, etc, actually sets the value to "" which results in URLs like http://:@example.com being used.

Steps to reproduce

• Set up pull mirroring with a username and password
• Unset the username and password
• Observe the generated URL and that mirroring fails as a result

What is the current bug behavior?

http://:@example.com

What is the expected correct behavior?

http://example.com

Possible fixes

Delete the key from the credentials hash if the value isn't present?