Fix Issue #400 - add test for passing file to fs.promise.readdir() (#409)

* Fix Issue #400 - add test for passing file to fs.promise.readdir()

This adds a test to ensure that `fs.promise.readdir()` throws an error when passing in a file.

* Made changes to `fsPromises.readdir` to be more promise-friendly

tests/spec/fs.readdir.spec.js

@@ -55,3 +55,24 @@ describe('fs.readdir', function() {
     });
   });
 });
+
+/**
+ * fsPromises tests
+ */
+
+describe('fsPromises.readdir', function() {
+  beforeEach(util.setup);
+  afterEach(util.cleanup);
+
+  it('should return an error if the path is a file', function() {
+    var fsPromises = util.fs().promises;
+
+    return fsPromises.open('/myfile', 'w')
+      .then(fd => fsPromises.close(fd))
+      .then(() => fsPromises.readdir('/myfile'))
+      .catch(error => {
+        expect(error).to.exist;
+        expect(error.code).to.equal('ENOTDIR');
+      });
+  });
+});