Add test that shows that rejectWith promises generates warnings

This test demonstrates that promises are created at configuration time
and not when the mock is called.

From docs:
  The 'rejectionHandled' event is emitted whenever a Promise has been
  rejected and an error handler was attached to it ... later than one
  turn of the Node.js event loop.
  ...
  The 'unhandledRejection' event is emitted whenever a Promise is
  rejected and no error handler is attached to the promise within a
  turn of the event loop.
  https://nodejs.org/api/process.html#process_event_rejectionhandled

test.js

@@ -994,6 +994,34 @@ describe('simple', function () {
       })
     })
 
+    describe('when mock rejectsWith', function () {
+
+      it('and mock is called at a later time then no UnhandledPromiseRejectionWarning and PromiseRejectionHandledWarning occurs', function (done) {
+        var warnings = []
+        function logWarning () {
+          var message = arguments.length === 2 ? 'Unhandled promise rejection' : 'Promise rejection was handled asynchronously'
+          warnings.push(message)
+        }
+        process.on('rejectionHandled', logWarning)
+        process.on('unhandledRejection', logWarning)
+
+        var mock = simple.mock().rejectWith(new Error('from rejectWith'))
+        setTimeout(function () {
+          mock().then(function () {
+            return Promise.reject('Mock should have been rejected')
+          }, function () {
+            assert.equal(warnings.length, 0, 'Warnings "' + warnings.join('", "') + '"')
+          })
+          .then(done, done)
+          .then(function () {
+            process.removeListener('rejectionHandled', logWarning)
+            process.removeListener('unhandledRejection', logWarning)
+          })
+        })
+      })
+
+    })
+
     describe('#noLoop', function () {
 
       it('should disable looping', function () {