stream: make Writable#end() accept a callback function

This is more backwards-compatible with stream1 streams like `fs.WriteStream`
which would allow a callback function to be passed in as the only argument.

Closes #4719.

lib/_stream_writable.js

@@ -289,20 +289,33 @@ Writable.prototype._write = function(chunk, cb) {
   });
 };
 
-Writable.prototype.end = function(chunk, encoding) {
+Writable.prototype.end = function(chunk, encoding, cb) {
   var state = this._writableState;
 
   // ignore unnecessary end() calls.
   if (state.ending || state.ended || state.finished)
     return;
 
+  if (typeof chunk === 'function') {
+    cb = chunk;
+    chunk = null;
+    encoding = null;
+  } else if (typeof encoding === 'function') {
+    cb = encoding;
+    encoding = null;
+  }
+
   state.ending = true;
   if (chunk)
-    this.write(chunk, encoding);
+    this.write(chunk, encoding, cb);
   else if (state.length === 0 && !state.finishing && !state.finished) {
     state.finishing = true;
     this.emit('finish');
     state.finished = true;
+    if (cb) process.nextTick(cb);
+  } else if (cb) {
+    this.once('finish', cb);
   }
+
   state.ended = true;
 };

test/simple/test-stream2-writable.js

@@ -251,3 +251,32 @@ test('write callbacks', function (t) {
   });
   tw.end();
 });
+
+test('end callback', function (t) {
+  var tw = new TestWriter();
+  tw.end(function () {
+    t.end();
+  });
+});
+
+test('end callback with chunk', function (t) {
+  var tw = new TestWriter();
+  tw.end(new Buffer('hello world'), function () {
+    t.end();
+  });
+});
+
+test('end callback with chunk and encoding', function (t) {
+  var tw = new TestWriter();
+  tw.end('hello world', 'ascii', function () {
+    t.end();
+  });
+});
+
+test('end callback after .write() call', function (t) {
+  var tw = new TestWriter();
+  tw.write(new Buffer('hello world'));
+  tw.end(function () {
+    t.end();
+  });
+});