Two fixes in the Basic Auth middleware

The first commit updates the middleware to access the new hash table representation of HTTP headers. The second commit is a bugfix for when passwords contain one or more `:` characters.

When `parse-user-and-pass` used `split-sequence` and the password contained colons the `destructuring-bind` in the `call` method failed with the wrong arity. Colons are allowed in passwords but not userids as per [RFC 2617](https://tools.ietf.org/html/rfc2617#section-2).