Protocol: Relax UTF-8 checking when sending.

All messages sent with `Protocol\Rfc6455::send` must be UTF-8 encoded.
This is a hard constraint. All messages received by the server must be
UTF-8 encoded, so we ease the work of the server and avoid sending
invalid messages by adding this constraint.

This constraint was checked when the opcode was either
`OPCODE_TEXT_FRAME` or `OPCODE_CONTINUATION_FRAME`. This is wrong
because in some cases a valid UTF-8 message can be send byte after byte
as fragments. Each fragment is not necessarily a valid UTF-8 string.

So now, this constraint is checked only when the opcode is
`OPCODE_TEXT_FRAME` and when the message is not fragmented (so `end` is
set to `true`). And so, this is possible to send a valid UTF-8 message
as many invalid UTF-8 fragments.

Protocol/Rfc6455.php

@@ -317,8 +317,8 @@ public function send(
         $end    = true,
         $mask   = false
     ) {
-        if ((Websocket\Connection::OPCODE_TEXT_FRAME         === $opcode ||
-             Websocket\Connection::OPCODE_CONTINUATION_FRAME === $opcode) &&
+        if (Websocket\Connection::OPCODE_TEXT_FRAME === $opcode &&
+            true  === $end &&
             false === (bool) preg_match('//u', $message)) {
             throw new Websocket\Exception\InvalidMessage(
                 'Message “%s” is not in UTF-8, cannot send it.',