Fix fs.realpathSync('/')

lib/fs.js

@@ -519,8 +519,10 @@ function realpathSync (p) {
     i = 0;
     buf = [''];
   }
-  return buf.join('/');
+  return buf.join('/') || '/';
 }
+
+
 function realpath (p, cb) {
   if (p.charAt(0) !== '/') {
     p = path.join(process.cwd(), p);

test/simple/test-fs-realpath.js

@@ -260,6 +260,15 @@ function runNextTest(err) {
 }
 runNextTest();
 
+
+assert.equal('/', fs.realpathSync('/'));
+fs.realpath('/', function (err, result) {
+  assert.equal(null, err);
+  assert.equal('/', result);
+});
+
+
+
 process.addListener("exit", function () {
   unlink.forEach(function(path){ try {fs.unlinkSync(path);}catch(e){} });
   assert.equal(async_completed, async_expected);