fix: fixing rpc return values for unity 2021.2 (#1000)

making Skeleton method static and adding the behaviour as the first arg.

Previously method was an instance method,. and behaviour was assumed to be first arg, this seemed to work in older versions of unity but has broken in 2021.2 when the method has a return value

Assets/Mirage/Weaver/Processors/ServerRpcProcessor.cs

@@ -176,19 +176,21 @@ private void CallSendServerRpc(MethodDefinition md, ILProcessor worker)
         MethodDefinition GenerateSkeleton(MethodDefinition method, MethodDefinition userCodeFunc, ValueSerializer[] paramSerializers)
         {
             MethodDefinition cmd = method.DeclaringType.AddMethod(SkeletonPrefix + method.Name,
-                MethodAttributes.Family | MethodAttributes.HideBySig,
+                MethodAttributes.Family | MethodAttributes.HideBySig | MethodAttributes.Static,
                 userCodeFunc.ReturnType);
 
+            _ = cmd.AddParam(method.DeclaringType, "behaviour");
             ParameterDefinition readerParameter = cmd.AddParam<NetworkReader>("reader");
             ParameterDefinition senderParameter = cmd.AddParam<INetworkPlayer>("senderConnection");
             _ = cmd.AddParam<int>("replyId");
 
 
             ILProcessor worker = cmd.Body.GetILProcessor();
 
-            // setup for reader
+            // load `behaviour.`
             worker.Append(worker.Create(OpCodes.Ldarg_0));
 
+            // read and load args
             ReadArguments(method, worker, readerParameter, senderParameter, false, paramSerializers);
 
             // invoke actual ServerRpc function