Returns the payload size of the body in octets if possible.
boost::optional< std::uint64_t > payload_size() const;
This function invokes the Body algorithm to measure
the number of octets in the serialized body container. If there is no body,
this will return zero. Otherwise, if the body exists but is not known ahead
boost::none is returned (usually indicating
that a chunked Transfer-Encoding will be used).
The value of the Content-Length field in the message is not inspected.