Support req.end with callback only

According to docs, `req.end` can accept callback as a first argument. That's what `got` module does.

Fixes nock#1509

```
request.end([data[, encoding]][, callback])#

History
data <string> | <Buffer>
encoding <string>
callback <Function>
Returns: <this>
Finishes sending the request. If any parts of the body are unsent, it will flush them to the stream. If the request is chunked, this will send the terminating '0\r\n\r\n'.

If data is specified, it is equivalent to calling request.write(data, encoding) followed by request.end(callback).

If callback is specified, it will be called when the request stream is finished.
```

lib/request_overrider.js

@@ -126,6 +126,10 @@ function RequestOverrider(req, options, interceptors, remove, cb) {
 
   req.end = function(buffer, encoding, callback) {
     debug('req.end')
+    if (_.isFunction(buffer) && arguments.length === 1) {
+      callback = buffer
+      buffer = null
+    }
     if (!req.aborted && !ended) {
       req.write(buffer, encoding, function() {
         if (typeof callback === 'function') {

tests/test_intercept.js

@@ -875,6 +875,38 @@ test('end callback called', t => {
   })
 })
 
+// https://github.com/nock/nock/issues/1509
+test('end callback called when end has callback, but no buffer', t => {
+  const scope = nock('http://example.test')
+    .post('/')
+    .reply(200, 'Hello World!')
+
+  let callbackCalled = false
+  const req = http.request(
+    {
+      host: 'example.test',
+      method: 'POST',
+      path: '/',
+      port: 80,
+    },
+    function(res) {
+      t.equal(callbackCalled, true)
+      t.equal(res.statusCode, 200)
+      res.on('end', function() {
+        scope.done()
+        t.end()
+      })
+      // Streams start in 'paused' mode and must be started.
+      // See https://nodejs.org/api/stream.html#stream_class_stream_readable
+      res.resume()
+    }
+  )
+
+  req.end(function() {
+    callbackCalled = true
+  })
+})
+
 // http://github.com/nock/nock/issues/139
 test('finish event fired before end event', t => {
   const scope = nock('http://example.test')