Clarify that header function receives the response body (#1544)

Closes #1542

README.md

@@ -526,14 +526,15 @@ const scope = nock('https://api.github.com')
 ```
 
 Or you can use a function to generate the headers values. The function will be
-passed the request, response, and body (if available). The body will be either a
-buffer, a stream, or undefined.
+passed the request, response, and response body (if available). The body will
+be either a buffer, a stream, or undefined.
 
 ```js
 const scope = nock('http://www.headdy.com')
   .get('/')
   .reply(200, 'Hello World!', {
-    'X-My-Headers': (req, res, body) => body.toString(),
+    'Content-Length': (req, res, body) => body.length,
+    ETag: () => `${Date.now()}`,
   })
 ```
 
@@ -556,9 +557,7 @@ Or you can use a function to generate the default headers values:
 ```js
 const scope = nock('http://www.headdy.com')
   .defaultReplyHeaders({
-    'Content-Length': function(req, res, body) {
-      return body.length
-    },
+    'Content-Length': (req, res, body) => body.length,
   })
   .get('/')
   .reply(200, 'The default headers should come too')

lib/request_overrider.js

@@ -494,7 +494,7 @@ function RequestOverrider(req, options, interceptors, remove, cb) {
 
       // Evaluate functional headers.
       const evaluatedHeaders = {}
-      Object.entries(response.headers).forEach(function([key, value]) {
+      Object.entries(response.headers).forEach(([key, value]) => {
         if (typeof value === 'function') {
           response.headers[key] = evaluatedHeaders[key] = value(
             req,

tests/test_reply_headers.js

@@ -146,10 +146,6 @@ test('reply header function receives the correct arguments', async t => {
       'X-My-Headers': (req, res, body) => {
         t.type(req, OverriddenClientRequest)
         t.type(res, IncomingMessage)
-        // TODO The current behavior is to pass the response body as a buffer.
-        // This doesn't seem at all helpful and seems like it is probably a
-        // bug.
-        // https://github.com/nock/nock/issues/1542
         t.type(body, Buffer)
         t.true(Buffer.from('boo!').equals(body))
         return 'gotcha'