Allow to replace default execute function (#391)

This PR allows providing your own `execute` function instead of the default one from the `graphql-js`.
It’s a small change that adds a lot of flexibility since you can wrap standard `execute` and it opens new possibilities for middlewares. 

Personally, I need it to proxy calls to 3rd-party APIs.
It’s an alternative to #253 but introduces less code and more flexible mechanism.
It’s very important for us as it will be the last change required to switch [graphql-faker](https://github.com/APIs-guru/graphql-faker) from custom forks of `graphql-js` and `graphql-express`.
@wincent @leebyron Can you please review it?

README.md

@@ -90,6 +90,9 @@ The `graphqlHTTP` function accepts the following options:
   * **`customValidateFn`**: An optional function which will be used to validate
     instead of default `validate` from `graphql-js`.
 
+  * **`customExecuteFn`**: An optional function which will be used to execute
+    instead of default `execute` from `graphql-js`.
+
   * **`customFormatErrorFn`**:  An optional function which will be used to format any
     errors produced by fulfilling a GraphQL operation. If no function is
     provided, GraphQL's default spec-compliant [`formatError`][] function will be used.

src/__tests__/http-test.js

@@ -29,6 +29,7 @@ import {
   GraphQLError,
   BREAK,
   validate,
+  execute,
 } from 'graphql';
 import graphqlHTTP from '../';
 
@@ -2000,6 +2001,37 @@ describe('test harness', () => {
       });
     });
 
+    describe('Custom execute', () => {
+      it('allow to replace default execute.', async () => {
+        const app = server();
+
+        let seenExecuteArgs;
+
+        get(
+          app,
+          urlString(),
+          graphqlHTTP(() => {
+            return {
+              schema: TestSchema,
+              async customExecuteFn(args) {
+                seenExecuteArgs = args;
+                const result: any = await Promise.resolve(execute(args));
+                result.data.test2 = 'Modification';
+                return result;
+              },
+            };
+          }),
+        );
+
+        const response = await request(app).get(urlString({ query: '{test}' }));
+
+        expect(response.text).to.equal(
+          '{"data":{"test":"Hello World","test2":"Modification"}}',
+        );
+        expect(seenExecuteArgs).to.not.equal(null);
+      });
+    });
+
     describe('Custom result extensions', () => {
       it('allows for adding extensions', async () => {
         const app = server();

src/index.js

@@ -20,6 +20,7 @@ import {
   getOperationAST,
   specifiedRules,
 } from 'graphql';
+import type { ExecutionArgs, ExecutionResult } from 'graphql';
 import httpError from 'http-errors';
 import url from 'url';
 
@@ -51,6 +52,7 @@ export type Options =
     ) => OptionsResult)
   | OptionsResult;
 export type OptionsResult = OptionsData | Promise<OptionsData>;
+
 export type OptionsData = {
   /**
    * A GraphQL schema from graphql-js.
@@ -88,6 +90,12 @@ export type OptionsData = {
     rules: $ReadOnlyArray<any>,
   ) => $ReadOnlyArray<GraphQLError>,
 
+  /**
+   * An optional function which will be used to execute instead of default `execute`
+   * from `graphql-js`.
+   */
+  customExecuteFn?: ?(args: ExecutionArgs) => Promise<ExecutionResult>,
+
   /**
    * An optional function which will be used to format any errors produced by
    * fulfilling a GraphQL operation. If no function is provided, GraphQL's
@@ -176,6 +184,7 @@ function graphqlHTTP(options: Options): Middleware {
     let pretty;
     let formatErrorFn = formatError;
     let validateFn = validate;
+    let executeFn = execute;
     let extensionsFn;
     let showGraphiQL;
     let query;
@@ -302,15 +311,15 @@ function graphqlHTTP(options: Options): Middleware {
         }
         // Perform the execution, reporting any errors creating the context.
         try {
-          return execute(
+          return executeFn({
             schema,
-            documentAST,
+            document: documentAST,
             rootValue,
-            context,
-            variables,
+            contextValue: context,
+            variableValues: variables,
             operationName,
             fieldResolver,
-          );
+          });
         } catch (contextError) {
           // Return 400: Bad Request if any execution context errors exist.
           response.statusCode = 400;
@@ -407,6 +416,7 @@ function graphqlHTTP(options: Options): Middleware {
         }
 
         validateFn = optionsData.customValidateFn || validateFn;
+        executeFn = optionsData.customExecuteFn || executeFn;
         formatErrorFn =
           optionsData.customFormatErrorFn ||
           optionsData.formatError ||