fix escape problem

With select option contains html escaped text, bootstrap-multiselect will unsecape it and write unescaped text directory.

Test case:
 http://hanzubon.jp/tmp/bootstrap-multiselect-test/escape_test.html

One of a select element contains `&lt;script&gt;alert('bbb')&lt;/script&gt; `

case 1:
 When read above page, alert box with text "bbb" will popup.

case 2:
 When select empty item (the select item contains above escaped text), alert box with text "bbb" will popup.