CommonHttpHeader and other similar enums should not exist. Enums make dealing with constant values more difficult. We're always calling toString on everything anyway, so we should just make them Strings. Also, HeaderConstant, wtf.
- Get rid of the HeaderConstant interface.
- Update all of the HeaderConstant implementation Enums to be Classes with String constants.
- Consolidate ExtendedHttpHeader and some of the other useless header enums to somewhere else.
- Ensure header equality continues to work (case insensitive).