WIP: encoding two different header sets should remove non-present headers
Encoding removes headers from reference set not in header set
Refactor reference set to have map implementation
Decode now removes from reference set
Add tests I really should have written
Fix bug that inadvertantly applied context update to indexed field
Refactor header field to use pointers, eviction works for decode
Add final decoding test from IETF draft
Merge pull request #5 from tildedave/fix_encoding_to_work_with_differential_headers

encoding two different header sets should remove non-present headers