Undo is disabled when a value in QDoubleSpinBox is changed

When a value in a QDoubleSpinBox is changed to a value containing fewer decimal points that its precision is set to, the undo history is cleared.

To reproduce this, run the Qt spinbox example (examples/widgets/spinboxes) and follow these steps:

1 - In the double precision spinboxes group, locate the "Enter a price..." spinbox.
2 - Change this value to 11.11 and press enter.
3 - Again, change this value to 22.22 and press enter.
4 - Again, change this value to 33.33 and press enter.
5 - Now press Ctrl+Z/Ctrl+Y to traverse through the undo/redo history of the spinbox. Notice that it appears to work fine
6- Now change the value to simply 4 and press enter.
7 - Once again, try to traverse through the undo/redo history of the spinbox. Notice that you are not able to undo any longer. Furthermore, right-clicking on the spinbox reveals that the undo entry is disabled.