Add a couple more empty/folding header tests.
Parse empty Accept headers as [].

RFC 3261 20.1: An empty Accept header field means that no formats are acceptable.
Parse empty Accept-Encoding as equivalent to "identity".

RFC 3261 20.2
  An empty Accept-Encoding header field is permissible.  It is equivalent to
  Accept-Encoding: identity, that is, only the identity encoding, meaning no
  encoding, is permissible.
Parse empty Supported headers as [].

RFC 3261 20.37
  If empty, it means that no extensions are supported.
Parse empty Allow headers as [].
Parse empty Accept-Language headers as [].