x/net/http2: if (*ClientConn).encodeHeaders returns an error, roundTrip doesn't clean up streams map, etc

[bradfitz]

If you send try to RoundTrip a bogus request that trips up (*ClientConn).encodeHeaders, the (*ClientConn).roundTrip function returns without cleaning up the cc.streams map, etc.

/cc @neild

[mknyszek]

CC @neild