Fixed async fs writes with length 0, it should fire the callback

lib/fs.js

@@ -211,7 +211,15 @@ fs.write = function (fd, buffer, offset, length, position, callback) {
     offset = 0;
     length = buffer.length;
   }
-  if(!length) return;
+
+  if (!length) {
+    if (typeof callback == 'function') {
+      process.nextTick(function() {
+        callback(undefined, 0);
+      });
+    }
+    return;
+  }
 
   binding.write(fd, buffer, offset, length, position, callback || noop);
 };

test/simple/test-fs-write.js

@@ -11,7 +11,7 @@ fs.open(fn, 'w', 0644, function (err, fd) {
   if (err) throw err;
   console.log('open done');
   fs.write(fd, '', 0, 'utf8', function(err, written) {
-      assert.fail('zero length write should not go through to callback');
+    assert.equal(0, written);
   });
   fs.write(fd, expected, 0, "utf8", function (err, written) {
     console.log('write done');