Use functions to generate header values.

This change provides the ability to generate header values and
default header values by evaluating a function. The function is
passed the request, response, and body (if available). The return
value is used as the header value.

Closes nock#263.

Author: jagoda

README.md

@@ -329,6 +329,20 @@ var scope = nock('http://www.headdy.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.
+
+```js
+var scope = nock('http://www.headdy.com')
+   .get('/')
+   .reply(200, 'Hello World!', {
+     'X-My-Headers': function (req, res, body) {
+       return body.toString();
+     }
+   });
+```
+
 ### Default Reply Headers
 
 You can also specify default reply headers for all responses like this:
@@ -343,6 +357,19 @@ var scope = nock('http://www.headdy.com')
   .reply(200, 'The default headers should come too');
 ```
 
+Or you can use a function to generate the default headers values:
+
+```js
+var scope = nock('http://www.headdy.com')
+  .defaultReplyHeaders({
+    'Content-Length': function (req, res, body) {
+      return body.length;
+    }
+  })
+  .get('/')
+  .reply(200, 'The default headers should come too');
+```
+
 ### Including Content-Length Header Automatically
 
 When using `scope.reply()` to set a response body manually, you can have the

lib/mixin.js

@@ -1,17 +1,14 @@
 'use strict';
-
-function clone(o) {
-	return JSON.parse(JSON.stringify(o));
-}
+var _ = require("lodash");
 
 function mixin(a, b) {
 	if (! a) { a = {}; }
 	if (! b) {b = {}; }
-	a = clone(a);
+	a = _.cloneDeep(a);
 	for(var prop in b) {
 		a[prop] = b[prop];
 	}
 	return a;
 }
 
-module.exports = mixin;
+module.exports = mixin;

lib/request_overrider.js

@@ -396,6 +396,15 @@ function RequestOverrider(req, options, interceptors, remove, cb) {
         authorized: true
       });
 
+      // Evaluate functional headers.
+      Object.keys(response.headers).forEach(function (key) {
+        var value = response.headers[key];
+
+        if (typeof value === "function") {
+          response.headers[key] = value(req, response, responseBody);
+        }
+      });
+
       process.nextTick(function() {
         var respond = function() {
           debug('emitting response');

tests/test_intercept.js

@@ -611,6 +611,24 @@ test("reply headers work with function", function(t) {
   });
 });
 
+test("reply headers as function work", function(t) {
+  var scope = nock('http://example.com')
+  .get('/')
+  .reply(200, 'boo!', {
+    'X-My-Headers': function (req, res, body) {
+      return body.toString();
+    }
+  });
+
+  var req = http.get({
+    host: 'example.com',
+    path: '/'
+  }, function (res) {
+    t.equivalent(res.headers, { 'x-my-headers': 'boo!' });
+    t.end();
+  });
+});
+
 test("match headers", function(t) {
   var scope = nock('http://www.headdy.com')
      .get('/')
@@ -1983,6 +2001,44 @@ test("default reply headers work", function(t) {
   }, done).end();
 });
 
+test("default reply headers as functions work", function(t) {
+  var date = (new Date()).toUTCString();
+  var message = 'A message.';
+
+  var scope = nock('http://default.reply.headers.com')
+    .defaultReplyHeaders({
+      'Content-Length' : function (req, res, body) {
+        return body.length;
+      },
+
+      'Date': function () {
+        return date;
+      },
+
+      'Foo': function () {
+        return 'foo';
+      }
+    })
+    .get('/')
+    .reply(200, message, {foo: 'bar'});
+
+  http.request({
+      host: 'default.reply.headers.com',
+      path: '/'
+    }, function (res) {
+      t.deepEqual(
+        res.headers,
+        {
+          'content-length': message.length,
+          'date': date,
+          'foo': 'bar'
+        }
+      );
+      t.end();
+    }
+  ).end();
+});
+
 test("JSON encoded replies set the content-type header", function(t) {
   var scope = nock('http://localhost')
     .get('/')