Upgrade to HPACK-09.

Tests should pass when #17 is merged.

Interface changes

- Header fields should be passed as an array of name-value pairs instead of a Hash.
  - However, the module still accepts a Hash for clients that do not care
    duplicative header names nor relative order between header fields.
- Compressor constructor accepts HPACK runtime optimization flags:
  - :table_size  Integer  maximum header table size in bytes
  - :huffman     Symbol   :always, :never, :shorter
  - :index       Symbol   :all, :static, :never
Given that https://github.com/igrigorik/http-2/pull/18/files#diff-4009761f111ccc84d99992ccac25135fR188 value is not used, is the 0/nil return necessary?