RUBY-608 fixing uri parser so that it handles authentication correctly

Looks like this has been broken for a while (I tested as far back as 1.7.1 and
this hasn't been working as advertised since at least then) and some of the
URI tests I added recently when we re-enabled the auth test suite weren't good
enough to catch this.

We were parsing the auth info out of the URI correctly, we just weren't
passing it to the client instance at all.

I've corrected this and as a result I also needed to address spec
violation/behavior discrepency in the URI parser that was discovered recently.

https://github.com/mongodb/mongo-ruby-driver/blob/master/lib/mongo/util/uri_parser.rb#L294-L295

While correcting this behavior issue is technically a correction to documented
behavior and the mongodb uri spec, it may be best to include this change in
the next minor release.