Allow specifying supported/preferred serializations in the client/server

[jrogers]

• Adds method to WampClientBuilder to add supported serializations
• Added overloaded constructors in server classes to supported specifying supported serializations
• Default serializers are still prioritized { JSON, MessagePack } if not specified

[Matthias247]

Hi. Sorry for the delay - was on vacation last week.

Do I understand it correctly that this is only for restricting the serialization on server or client side to a subset of all protocols? And not for also providing custom serialization methods (ObjectMappers) as we talked about in #25 ?

Most things are looking good to me. I would personally prefer a Set instead of a List for the Serializations and also having a static DefaultWampSerializations Set instead of abusing null for it, but both will work.

I think the WampServerWebsocketHandler really needs one more update:
serialization = WampSerialization.fromString(actualProtocol); will return any valid serialization even if it's not contained in supportedSerializations. We can't rely on Netty here to close the connection if the requested subprotocol is not in subProtocols - because the WebSocket spec says servers should accept connections even if they don't support it and send the subprotocol that they support back to the client.

So this

if (serialization == WampSerialization.Invalid) {

should be fixed into something like

if (serialization == WampSerialization.Invalid || !supportedSerializations.contains(serialization)) {

Ah and another small thing: WampClientBuilder should reject having WampSerialization.Invalid in the list. Like:

for (WampSerialization serialization : serializations) {
    if (serialization == WampSerialization.Invalid) throw new ApplicationError(ApplicationError.INVALID_SERIALIZATIONS);
    this.serializations.add(serialization);
}

[Matthias247]

Merged with some small changes (default List instead of null and better validation). Didn't go for Sets as I realized that the ordering of the Serializations is important.

[jrogers]

Sorry about not replying to or updating this one, I have been very busy at work lately. Thanks for the change.