HTTP/2: signal 0-byte HPACK's dynamic table size. This change lets NGINX talk to clients with SETTINGS_HEADER_TABLE_SIZE smaller than the default 4KB. Previously, NGINX would ACK the SETTINGS frame with a small dynamic table size, but it would never send dynamic table size update, leading to a connection-level COMPRESSION_ERROR. Also, it allows clients to release 4KB of memory per connection, since NGINX doesn't use HPACK's dynamic table when encoding headers, however clients had to maintain it, since NGINX never signaled that it doesn't use it. Signed-off-by: Piotr Sikora <piotrsikora@google.com>