Fix user:pass handling in AuthContext

ArrayIndexOutOfBoundsException was being thrown causing weird errors

Change-Id: I95c66a2c06300c58752269508449aef32c71d49b
Reviewed-on: http://review.couchbase.org/27823
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>