underlying send() function is free to send a subset of the bytes, and
occasionally does (especially on non-blocking sockets).  The return value
was true if all bytes were sent or false if not all bytes were sent.

The new behaviour is to return the number of bytes sent or null on failure.

This is mostly compatible with existing code which appears to universally use
if (!sock.send(str))

Cases where it's not compatible are when a zero-length string is sent.  The
handling of errors is slightly different now too... previously, if you
retransmitted on failure, the stream would "stutter" now it will have
"holes".