Check response code, make sure it's 101.
Pass http_message to the client to keep it appraised.
This represents a slight change in the API -
in case of an error MG_EV_WEBSOCKET_HANDSHAKE_DONE will now be delivered where previosuly connection would just hang.
Clients that do not examine the argument may for a moment think handshake has succeeded but in fact connection will be closed immediately.
CL: mg: Fix handling of WS handshake error response
PUBLISHED_FROM=645a43d9e5bee216e54411f85827c9b974e9a7d1
It can specify how much data was actually processed and the rest will be re-delivered on next poll.
CL: mg: Add ability for multipart data handler to provide pushback
PUBLISHED_FROM=e0168c5064c3a32921c9209bc09f4da2079cd616
