NumericUpDown HasDecimals Is Broken.

## What steps will reproduce this issue?

After issue #2149 the `HasDecimals` property became broken.
### Expected outcome

The `HasDecimals = false` blocks the user from typing `,` or `.`

I already have a fix, and will send the pull request soon.
## 
### Environment
- MahApps.Metro - Latest Master
- Windows **7x64**
- Visual Studio **2015**
- .NET Framework **4.5**
