Push data (if there is any) from client's m_send_buffer to the connected socket.
Declared in <httpserver.h>
[[requires_capability(0x7fbc93a5a3e8, 0x7fbc93a5a4a8)]]
bool
MaybeSendBytesFromBuffer();
false if we are done with this client and HTTPServer can skip the next read operation from it.