SPDY: detect premature closing of stream. The SPDY/3.1 specification requires that the server must respond with a 400 "Bad request" error if the sum of the data frame payload lengths does not equal the size of the Content-Length header. This also fixes "zero size buf in output" alert, that might be triggered if client sends a greater than zero Content-Length header and closes stream using the FIN flag with an empty request body.