fix: properly handle full closing handshake

[mccutchen]

Right now, when our websocket server initiates the closing handshake, it does not wait for the client to reply before closing the connection, which is a (mild?) violation of the protocol.

Bonus mystery: It seems like the autobahn test suite should probably cover this, but we're passing those tests with our incomplete implementation. Could this be related to the mystery of why the server never seems to actually close the underlying TCP connection in our test suite?

websocket/websocket_test.go

Lines 325 to 331 in aa7a462

	// FIXME: this write should fail, but the connection doesn't seem to
	// get closed. for now we skip.
	if false {
	wn, err := conn.Write([]byte("foo"))
	assert.Equal(t, wn, 0, "expected to write 0 bytes")
	assert.Error(t, err, net.ErrClosed)
	}