Use crypto:hash/2 function instead of ones from p1_sha.
This function exists since commit
erlang/otp@208f9ad382 and also implemented
as NIF, so I believe it's safe to use it.
Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
RFC 6455 says that the client's opening handshake includes an Upgrade
header field "containing the value 'websocket', treated as an ASCII
case-insensitive value."
Closes#510.