Fix and test multiple select repopulation.

This PR fixes #202, #99 and possibly #221 and #273.

It ends up that the Nested models population code in the Select constructor was breaking multiselect repopulation for options with string values.

I added additional test cases for options with string values.

I'm hopeful that the `str_contains($this->name, '.')` check I added is acceptable, as this was by far the cleanest fix I found. Can anyone verify that the "Multiple models population" code in Select::__construct() should only be needed if the name contains a dot?