refactor(hooks)!: Async hooks must return false when sending a response

BREAKING CHANGE: Async hooks that send a response must now return `false`. If a hook sends a response but does not return `false`, the next hooks and handler will be run, and res.send() will probably be called twice, resulting in an error.

docs/Hooks.md

@@ -86,9 +86,15 @@ app.addHook('onRequest', (req, res, next) => {
 
 If sending a response from inside a hook, **`next()` must not be called**.
 
-If sending a response from inside an `async` hook, **`res.send()` must be used**.
-If it is not, Medley will not know that a response has been sent, so the hooks
-will continue running.
+If sending a response from inside an `async` hook, **the hook must return
+`false`** to prevent Medley from continuing on to run the next hooks.
+
+```js
+app.addHook('onRequest', async (req, res) => {
+  res.send({ early: 'response' });
+  return false;
+});
+```
 
 ### Handling Errors
 
@@ -105,19 +111,19 @@ app.addHook('onRequest', (req, res, next) => {
       next(err);
       return;
     }
+    // ...
   });
 });
 ```
 
 Async-await/promise errors are automatically caught and handled by Medley.
 
 ```js
-const fs = require('fs');
-const util = require('util');
-const readFile = util.promisify(fs.readFile);
+const fs = require('fs').promises;
 
 app.addHook('onRequest', async (req, res) => {
   const buffer = await fs.readFile('./someFile.txt');
+  // ...
 });
 ```
 
@@ -164,7 +170,7 @@ app.addHook('onSend', (req, res, payload, next) => {
 });
 ```
 
-To modify the payload using an `async` hook, return the new payload:
+To modify the payload using an `async` hook, return the new payload.
 
 ```js
 app.addHook('onSend', async (req, res, payload) => {

lib/HookRunners.js

@@ -31,8 +31,8 @@ const HookRunners = {
       }
     }
 
-    function handleResolve() {
-      if (!res.sent) {
+    function handleResolve(value) {
+      if (value !== false) {
         next(null)
       }
     }

test/hooks-async.test.js

@@ -102,6 +102,7 @@ test('onRequest hooks should be able to send a response', (t) => {
 
   app.addHook('onRequest', async (req, res) => {
     res.send('hello')
+    return false
   })
 
   app.addHook('onRequest', async () => {

test/hooks.test.js

@@ -601,7 +601,7 @@ test('async onRequest hooks should be able to send a response', (t) => {
 
   app.addHook('onRequest', (req, res) => {
     res.send('hello')
-    return Promise.resolve()
+    return Promise.resolve(false)
   })
 
   app.get('/', () => {