It would be desired to add option to make this limit configurable.
Fix scripted_recv_callback in tests.

The validation rule is described in
http://tools.ietf.org/html/draft-ietf-httpbis-header-compression-02#section-4.1.2

Instead of emitting common headers on decoder side, encoder
now keeps track of removed common headers and encode it
as necessary.

Following cases are now treated as connection error of type
PROTOCOL_ERROR.

* Receiving HEADERS from client in reserved (local)
* Receiving PUSH_PROMISE against peer-initiated stream
* Receiving WINDOW_UPDATE against in reserved (local, remote)