Minecarts do not update the OnGround tag to initiate a new snap

Steps To Reproduce:

1. Enable the Experimental Minecarts features

2. Set up any of the shown rail configurations (this may be directional or require specific rail placement techniques to set up. Using a debug stick is an easy alternative if configuring the rails properly is found to be difficult)

3. Place a cart on the top left sloped rail

4. Allow the minecart to move forward into range above the lower rail (the range is dependent upon the center of the minecart bounding box being above the rail)

Observed Results:

When moving to a rail where it needs to snap down to the next rail, it does not update the OnGround tag to initiate it to snap to the next rail, however the cart can still be considered on the bottom rail as well, causing acceleration. This leads to the cart being both OnGround and considered on the rail below it, which can subsequently provide acceleration without being in contact. This also means the cart does not snap down through carpet in this set up, unlike when being dropped from above.

Expected Results: 

The minecart should snap down to the rail when it is in vertical range do so.  The minecart should not float, nor should it be able to be impacted by the rail below it while floating above said rail.