PreSelected color bug fix

This PR addresses a bug causing the preSelected color to be retained when the segment is unselected

#### Steps to reproduce:
1. Touch down inside of a segment
2. Drag to another segment without leaving the THSegmentedControl frame
3. Let go

#### Result:
The originally touched segment turns the preSelected color and not return to the initial segmentBackgroundColor

#### Fix:
Reset background color when `changeSelectorToPreSelectionColor` is called and the current segment is not preSelected