fix issue#392-- add test for promises.rename() to rename existing file

tests/spec/fs.rename.spec.js

@@ -151,4 +151,44 @@ describe('fs.rename', function() {
       });
     });
   });
+
+  it('(promise version) should rename an existing file', function(done) {
+    var complete1 = false;
+    var complete2 = false;
+    var fs = util.fs();
+
+    function maybeDone() {
+      if(complete1 && complete2) {
+        done();
+      }
+    }
+
+    fs.open('/myfile', 'w+', function(error, fd) {
+      if(error) throw error;
+
+      fs.close(fd, function(error) {
+        if(error) throw error;
+
+        fs.promises.rename('/myfile', '/myotherfile').then(
+          function(){
+            fs.stat('/myfile', function(error, result) {
+              expect(error).to.exist;
+              expect(result).not.to.exist;
+              complete1 = true;
+              maybeDone();
+            });
+      
+            fs.stat('/myotherfile', function(error, result) {
+              expect(error).not.to.exist;
+              expect(result.nlinks).to.equal(1);
+              complete2 = true;
+              maybeDone();
+            });
+          },
+          function(error){throw error;}
+        );
+        
+      });
+    });
+  });
 });