Fixed bug with locked handlers

### Bug subject

When slider handles overlaps each other in min or max positions they zIndex calculates wrong. It mean that user can't split them. You can repeat this bug by moving left handle to the leftmost position, right handle to the leftmost position then move both handles to the rightmost position, in demo on your site.

P.S.
Unfortunatly my text editor removes all trailing spaces and tabs, so all code marked as changed. Described changes located on 271-277 and 394-397 lines.