ignore ping frame when connection is closed

[HoneyryderChuck]

while it's valid to process ping frames after sending/receiving goaway frames, preparing an AC frame may inadvertedly put bytes on the user buffer, thereby signaling that there's something to write back instead of terminating the connection,\

@igrigorik @mullermp would like to ask you guidance on this one. The RFC is not specific on what to do when receiving a PING frame after receiving a GOAWAY frame with stream id 0. I found at least one instance of a server which does exactly that, which led to some corruption on a long lived client connection, and while I can theoretically work around it on the client side in order to wipe out the buffer when processing a ping ack frame on a closed connection, that seems to be redundant and something the parser should do for me.