Support use of dataloader from suspend function

Rudimentary implementation to support returning a `CompletableFuture`
from a suspend function.

`CoroutinesUtils.invokeSuspendingFunction` wraps the return value of
the function in a `Mono` (or `Flux`). But it also does this when a
`CompletableFuture` is returned, and thus results in a
`Mono<CompletableFuture<?>>`, which isn't captured by graphql-java,
and thus the dataloader is never dispatched.

By unwrapping the future, and _converting_ it to a mono (as opposed
to wrapping), the dataloader is dispatched correctly.

See gh-653

Author: koenpunt

spring-graphql/src/main/java/org/springframework/graphql/data/method/InvocableHandlerMethodSupport.java

@@ -26,6 +26,7 @@
 
 import graphql.GraphQLContext;
 import io.micrometer.context.ContextSnapshot;
+import org.springframework.data.util.KotlinReflectionUtils;
 import reactor.core.publisher.Mono;
 
 import org.springframework.core.CoroutinesUtils;
@@ -81,7 +82,18 @@ protected Object doInvoke(GraphQLContext graphQLContext, Object... argValues) {
 		Method method = getBridgedMethod();
 		try {
 			if (KotlinDetector.isSuspendingFunction(method)) {
-				return CoroutinesUtils.invokeSuspendingFunction(method, getBean(), argValues);
+				Object result = CoroutinesUtils.invokeSuspendingFunction(method, getBean(), argValues);
+
+				Class<?> returnType = KotlinReflectionUtils.getReturnType(method);
+
+				if (CompletableFuture.class.isAssignableFrom(returnType)) {
+					@SuppressWarnings("unchecked")
+					Mono<CompletableFuture<?>> mono = (Mono<CompletableFuture<?>>)result;
+					// Unwrap nested CompletableFuture
+					return mono.flatMap(Mono::fromFuture);
+				}
+
+				return result;
 			}
 			Object result = method.invoke(getBean(), argValues);
 			return handleReturnValue(graphQLContext, result);