DatePicker padding do not work

## What steps will reproduce this issue?

`<DatePicker Padding="10"/>` not working, but wpf default style works.

### Expected outcome

Expect the same logic as the WPF default style


### Environment

- MahApps.Metro __v1.5.0.23__
- Windows OS __Win7 64Bit SP1__
- Visual Studio __2015__
- .NET Framework __4.0__
