Fix for the FlatSlider changes made by @HansHuang
Merge pull request #1034 from Amrykid/fix-flatsider-35c75ed6600881ef3bd121c11f371c4057797c78

Fix for the FlatSlider changes made by @HansHuang
Merge branch 'master' of https://github.com/MahApps/MahApps.Metro into feature/RangeSlider