Ensure form object is passed down to InstanceTag

The `country_select` and `subregion_select` methods on
`ActionView::Helpers::FormBuilder` do not update the `options` hash with
the `:object` key, resulting in the value of the object passed into the
form builder not being selected in the HTML output.

Updated `FormBuilder` methods to add its object to the `options` hash and
cleaned up the logic for supporting the legacy API for priority
countries.

Add tests for these regressions and update the test suite to pass a
symbol instead of an `OpenStruct` instance when using the
`FormOptionsHelper` methods as this is the intended API for these
methods (based on what I can glean from the Rails APIs and code).