Upgrade to HPACK-09.

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
Simplify default option handling
Use :value for changetablesize parameter
Rspec 3.1.3 comes with rspec-autotest
Remove add_to_table return value.
Improve readability
Stop searching headers table when an exact match found
Add set_table_size delegation from Compressor/Decompressor to EncodingContext