Fix problem with positioning the dropdown when it is on the right hand side of view

In the stable version this line looks like:

dropLeft = offset.left + width - dropWidth;

In line (1193) the variable width is set to dropWidth which means we are always going to end up with 

dropLeft = offset.left + X - X
or
dropLeft = offset.left

Which means we are still aligned to the left of the button.

This current solution from @ef9f1dea is just aligning the select2-drop closer to the center which is not what it did before the dropdownAutoWidth.


The line I submitted worked perfect and my dropdown now aligns to the RHS of the button again as expected.
Merge pull request #1984 from jibwa/master

Fix problem with positioning the dropdown when it is on RHS