Fix #650

No longer attempting to add an ID if source select did not have one.
Added a test to account for this.

Author: mlh758

src/jquery.multiselect.js

@@ -1,6 +1,6 @@
 /* jshint forin:true, noarg:true, noempty:true, eqeqeq:true, boss:true, undef:true, curly:true, browser:true, jquery:true */
 /*
- * jQuery MultiSelect UI Widget 1.15pre
+ * jQuery MultiSelect UI Widget 1.16pre
  * Copyright (c) 2012 Eric Hynds
  *
  * http://www.erichynds.com/jquery/jquery-ui-multiselect-widget/
@@ -60,7 +60,7 @@
       var button = (this.button = $('<button type="button"><span class="ui-icon ui-icon-triangle-1-s"></span></button>'))
         .addClass('ui-multiselect ui-widget ui-state-default ui-corner-all')
         .addClass(o.classes)
-        .attr({ 'title':el.attr('title'), 'tabIndex':el.attr('tabIndex'), 'id': el.attr('id') + '_ms' })
+        .attr({ 'title':el.attr('title'), 'tabIndex':el.attr('tabIndex'), 'id': el.attr('id') ? el.attr('id')  + '_ms' : null })
         .prop('aria-haspopup', true)
         .insertAfter(el),