Add padding to select2 container displayed above

Added padding to ".select2-search" container instead of margin for ".select2-search input". This fixes bug of "#select2" movement when resizing page.

Steps to reproduce the bug:
- open select2 so it appears above the input
- resize the window

Actual results:
- #select2-drop moves up by 4px

Expected results:
- #select2-drop stays in same place
Consistent padding for .select2-search

Case: the dropdown should be shouwn above input. PositionDropdown function then operates by showing dropdown below (and  ".select2-search input" has no margin, it gets the wrong dropdown size). Then in the same function calculates the top value, but doesnt take into account, that input will have the "margin-top: 4px". Thus dropdown is displayed 4px lower.

Then the window resizes, the dropdown position is already correct, but then calculations (outerHeight()) get the height including the ".select2-search input" margin of 4px and thus moves the dropdown up.
Merge pull request #2770 from stonys/master

Add padding to select2 container displayed above