ShadowViewPager should not dispatch to the listener during setCurrentItem if the index to set is unchanged.

I had come across an issue where my listener was calling back to setCurrentItem(), causing a cyclical recursion and eventual stack overflow. With this fix, the second call to setCurrentItem in the recursion will result in a no-op, allowing the original call to complete.  This matches the logic in the real ViewPager class.
Merge remote-tracking branch 'bchang/master'

* bchang/master:
  ShadowViewPager should not dispatch to the listener during setCurrentItem if the index to set is unchanged.

Conflicts:
	src/main/java/com/xtremelabs/robolectric/shadows/ShadowViewPager.java