Allow auth with just username or just password

[yousefamar]

This PR makes two changes:

1. You can make requests that require authentication with just a username (no password) or just a password (no username). This seems to be possible pretty much everywhere else (including browsers) and I couldn't find anything in W3C docs that say otherwise (please correct me if I'm wrong). The reason I put this in was to be able to use VLC's http interface that needs a password but with no username.
2. Originally, if no password was given, the password variable would be set to null. Later, it would be set to "" if its type was undefined. Clearly the password is never undefined because the typeof null is object. Thus, with no password, your auth info would be "bob:null" instead of "bob:" as it should be. This was fixed and the same was done for usernames to make the first change work.