Add a test for #199

Author: wravery

samples/client/mutate/mutate.today.graphql

@@ -1,13 +1,13 @@
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # Licensed under the MIT License.
 
-mutation CompleteTaskMutation($input: CompleteTaskInput! = {id: "ZmFrZVRhc2tJZA==", isComplete: true, clientMutationId: "Hi There!"}) {
+mutation CompleteTaskMutation($input: CompleteTaskInput! = {id: "ZmFrZVRhc2tJZA==", isComplete: true, clientMutationId: "Hi There!"}, $skipClientMutationId: Boolean!) {
   completedTask: completeTask(input: $input) {
     completedTask: task {
       completedTaskId: id
       title
       isComplete
     }
-    clientMutationId
+    clientMutationId @skip(if: $skipClientMutationId)
   }
 }

src/ClientGenerator.cpp

@@ -608,7 +608,10 @@ response::Value serializeVariables(Variables&& variables)
 			sourceFile << R"cpp(	result.emplace_back(R"js()cpp" << variable.name
 					   << R"cpp()js"s, ModifiedVariable<)cpp";
 
-			if (_schemaLoader.getSchemaType(variable.type->name()) != SchemaType::Scalar)
+			const auto& builtinTypes = _schemaLoader.getBuiltinTypes();
+
+			if (builtinTypes.find(variable.type->name()) == builtinTypes.cend()
+				&& _schemaLoader.getSchemaType(variable.type->name()) != SchemaType::Scalar)
 			{
 				sourceFile << R"cpp(Variables::)cpp";
 			}