Fixed GamePadDPad Construction

This PR fixes the constructor for `GamePadState` to properly set the `GamePadDPad` button state.

Fixes #3070.
@danzel @thefiddler @nezz @KonajuGames 

As part of this PR I started adding a test for the trigger state and found that we're not setting things right.  

It seems XNA applies a deadzone to the left/right triggers to decide if `IsButtonDown` returns true or not.  The test above passes on XNA which suggests the deadzone is calculated within the `GamePadState` constructor or within `IsButtonDown/Up` itself.

In MonoGame it seems we've left this deadzone calculation to the `PlatformGetState` call only.

The first question is should the deadzone calculations be platform specific or common across all the platforms?