So the delegate can close the socket instead of keeping sending data.
Read EOF or h2 half-closed (remote) state was introduced in
https://codereview.chromium.org/129543002. But StreamSocket doesnt
really supports a half closed state, so upon a read EOF the only sane
action is to close the socket immediately even if in theory more send
is possible.
Per RFC 7540#6.4:
However, after sending the RST_STREAM, the sending endpoint MUST be
prepared to receive and process additional frames sent on the stream
that might have been sent by the peer prior to the arrival of the
RST_STREAM.
