Fix multiple selected detection with optgroup

[2j0ye]

Based on the work of @ruoqianfengshao
This is a basic change that fixes the issue #217
Fork based on master / v1.13.0

• I didn't upgrade any npm dependencies
• I just changed the line 406 of the file src/jquery.selectric.js as shown below

src/jquery.selectric.js
@@ -403,7 +403,7 @@
-  selectedIndex.push($(this).index());
+  selectedIndex.push($(_this.$element.find('option')).index(this));

• And re-ran the build via gulp

Up to you, this fork seems to be OK.
Julien

[codecov-io]

Codecov Report

Merging #223 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #223   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         391    391           
=====================================
  Hits          391    391

Impacted Files	Coverage Δ
src/jquery.selectric.js	100% <100%> (ø)	⬆️

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