Fix: in the WebSocket Client add-on, fix the base64 encoding

[Yves57]

When the number of bytes to encode is not a multiple of 3, the implementation of _nx_utility_base64_encode() uses an extra byte in the input buffer name that must equal to 0 (but this extra-byte must not be counted in name_size).

You can see a comment about that in this source code for example.

The bug causes the WebSocket handshake to fail randomly.

The fix has been double-checked with the following Go code as Base64 encoding reference:

func main() {
	digest := []byte{122, 200, 1, 65, 118, 43, 209, 42, 58, 252, 232, 253, 244, 242, 66, 67, 148, 135, 185, 105}
	key := base64.StdEncoding.EncodeToString(digest)
	fmt.Println(key)
}