Fixed a bug in which the selection was not made even if it was selected again.

[zaru]

Thanks for the great library.

Fixed a bug that when the multiple option was enabled, it was not enabled even if the choice was selected again.

Steps to Reproduce

• Set multiple option <select multiple="multiple">
• Click on an option
  • document.querySelector('select').selectedOptions.length equals 1
• Click on an option
  • document.querySelector('select').selectedOptions.length equals 0
• Click on a choice and select again
  • document.querySelector('select').selectedOptions.length equals 0 // NG, expect 1