Restore ability to insert columns at other positions than end of columns array

I insert a column definition at a specific position by using Array#splice() rather than Array#push(). However, the new column is rendered as the last column of the table instead of at the desired position.

This was already fixed by this commit: https://github.com/DJLemkes/ng-grid/commit/888c52ba7b683f4380e00740edc2df6d26287616#diff-f535df1f4b6fa3cc5ed32620fd6c6398R172
but it was later broken by this commit:
https://github.com/angular-ui/ng-grid/commit/5e79cad0a0ebacc88a3db58be8be231dd40553c1#diff-f535df1f4b6fa3cc5ed32620fd6c6398L249

This pull request restores the fix from the former commit.