Override options selected function to allow all boxes to be checked by default

This allows the attribute default_value to be set to 'checked' to set the default state of all boxes as checked.
It does not.  With a single checkbox, you can set an element as checked by setting it's default value equal to the checked value.  That isn't applicable here as it would set all the values of the checkboxes to the default which makes multi checkboxes useless.  You need a flag to specify that you want all values within the checkboxes to be checked.  Checked seemed the most intuitive to me.  Is there a better way to handle it?