Removed unnecessary appendData(), the Slim render() already does that.
Fixed unit tests

The unit test for rendering templates expected the old calls.  I removed
those expectations and changed the expectation on for Slim::render to
reflect the new way it is called.
Merge pull request #2 from yaworsw/render-optimization

Fixed unit tests
Merge pull request #18 from falmp/render-optimization

Removed unnecessary appendData(), the Slim render() already does that.