websocket-server: add client-handshake signal

This signal allow the application acting as server to decide whether
it accepts to denies a client's connection. To do so it is given the
resource asked by the client and the headers and should return a
SoupStatus code SWITCHING_PROTOCOLS to accept the connection, or any
other code to reject the connection.
message-headers: add contains() method
websocket: drop internal function in favor of soup_message_headers_contains()
message-headers: add equals() method
websocket: drop internal function in favor of soup_message_headers_equals()
message-headers: add empty() method
websocket: drop internal function in favor of soup_message_headers_empty()