consolidate AcceptEncodingXXXHeader classes into the single AcceptEncoding class

[mmerickel]

• Ensure AcceptEncoding instances are immutable.
• Merge missing/invalid/valid into the single instance - there were very very minor differences between the classes.
• Delete deprecated __iter__.
• Reimplement best_match, quality, and __contains__ methods using acceptable_offers.
  • A tuple can no longer be an offer containing server-side quality values.
  • An offer will only match a * clause in a header if it does not match
    any other clauses.
  • The identity offer was not properly considered a match unless the
    header explicitly it excluded via *;q=0 or identity;q=0.