Use content-box for text inputs

It's normally the default anyway but it's becoming very popular to set
* { box-sizing: content-box; } because it fixes so many annoying things.
Unfortunately, it can cause a problem when height and top or bottom
padding are set. In the case of this element, if border-box has been
set, we will only have 5px of vertical space left. So make sure it's
using content-box.
Merge pull request #2009 from okonomiyaki3000/text-input-box-sizing

Use content-box for text inputs
Fixing focus issue:

When disable_search flag is on, focus + hitting spacebar on the dropdown
causes the page to scroll and open the popup.
Adding preventDefault when the spacebar disables the scroll event to
happen.
Merge pull request #2046 from itayadler/master

Fixing focus issue:
Make all of Chosen use border-box
Merge pull request #2054 from harvesthq/chosen-box-sizing

Make all of Chosen use border-box