Fails to tolerate multiple dynamic table size updates in one block.

[Lukasa]

Spotted when running h2spec:

Generic tests for HTTP/2 server
  5. HPACK
         [send] SETTINGS Frame (length:6, flags:0x00, stream_id:0)
         [recv] SETTINGS Frame (length:36, flags:0x00, stream_id:0)
         [send] SETTINGS Frame (length:0, flags:0x01, stream_id:0)
         [recv] SETTINGS Frame (length:0, flags:0x01, stream_id:0)
         [send] HEADERS Frame (length:20, flags:0x05, stream_id:1)
Error decoding header block: Invalid table index 136381267
         [recv] GOAWAY Frame (length:8, flags:0x00, stream_id:0)
         [recv] Connection closed
    × 15: Sends multiple dynamic table size update
      -> The endpoint MUST accept multiple dynamic table size update
         Expected: HEADERS Frame (stream_id:1)
           Actual: Connection closed

Note the error after h2spec sends the HEADERS frame, leading to a GOAWAY.

The header block is the following Python byte string: b'?a?\xe1\x1f\x82\x87\x84A\x8a\x08\x9d\\\x0b\x81p\xdcy\xa6\x99'.

[Lukasa]

Ok, worked out the error. There was a subtle programming error in the code handling header compression context updates that meant that any update that was not literally first in a byte stream failed. This was mostly ok, as they should always be first, but it was possible to get more than one in a row and that would fail. Fix incoming.