try to fix issue#741

[rihitenLab]

Sorry for the ugly English.

I tried to fix the issue #741 .
This issue can happen frequently, so consider merging.

Cause

There are two cause for this issue:

1. In changeCurrentIndexProgressive in didSelectItemAt, OldCell is nil, because OldCell is off the screen.

2. cellForItemAt is called when an OldCell outside the screen enters the screen with moveTo.
   However, the currentIndex has not been updated, the OldCell is determined to be a NewCell.

Amendment policy

The fix was applied only inside the buttonBar related sources. Because of the following two reasons

1. We also considered modifying PagerTabStripViewController, but the scope of influence is too wide.

2. changeCurrentIndexProgressive exists only in buttonBar.

[rihitenLab]

I'm sorry.
I found an omission, so please wait for a review.