Fix execution with graphql-js 0.12.x

This fixes executing a query if all fields it encounters have synchronous resolvers, for which behavior changed in 0.12.x due to graphql/graphql-js#1115.

Closes #718, closes #720

Author: martijnwalraven

packages/apollo-server-core/src/runQuery.ts

@@ -152,15 +152,15 @@ function doRunQuery(options: QueryOptions): Promise<GraphQLResponse> {
 
     try {
         logFunction({action: LogAction.execute, step: LogStep.start});
-        return execute(
+        return Promise.resolve(execute(
             options.schema,
             documentAST,
             options.rootValue,
             context,
             options.variables,
             options.operationName,
             options.fieldResolver,
-        ).then(result => {
+        )).then(result => {
             logFunction({action: LogAction.execute, step: LogStep.end});
             logFunction({action: LogAction.request, step: LogStep.end});