WebSocketHandler: Added virtual OnInvokeMethodReceived for custom behaviour.

[Henry00IS]

This allows me to use the original library without having to hack logic into the source to for example prefix method calls and find the method in different classes compared to just one file (my WebSocketHandler).

Additionally:

• Prevented crash when the server receives non-json data.
• Added a bit of documentation to the virtual overrides.
• This is completely backwards compatible.

[Henry00IS]

Here is an example of why I need this to be virtual.

My client calls a method like this:

Which will arrive at my server's new processing method:

Now I can call my fancy methods in different classes depending on the prefix:

And yes this ᐅ character is awesome. I use it for when you have 0 references to indicate this is called through reflection. In case you were wondering. It also prevents people from calling methods I didn't intend to be called. Last time I had to change this entire library but this one virtual method makes it all possible.