Fixed ArrayController failed to recognize row 0 as the insert location

The logic is wrong in the test for last selection since it can be 0
and if there is no selection, [_selectionIndexes lastIndex] returns -1

Fixes #1723
added in the xib that I modified for testing.
Merge pull request #1856 from BlairDuncan/arrayControllerFixFor1723

Fixed ArrayController failed to recognize row 0 as the insert location
Fixed: more precise array controller insert index test.

Refs #1856.