Allow radio selection buttons that are checked

Selection buttons did not support radio buttons that were checked when the page loads because of an error with the order of code in the `getSelections` method. 

Each instance of the `RadioButtons` constructor has a `selections` property that is used to store a reference to the selected one for each group of radio buttons sharing the same name. The entries in `selections` are set in the `getSelections` method. The `markSelected` method is called on any radios already checked.

The previous order meant the `markSelected` method was called before the `selections` property was created. The `markSelected` method requires an entry in `selections` for the radio button you send in so this caused the method to fail.

This fix ensures `selections` has an entry for a radio button before calling `markSelected` on it and includes `markSelected` in the test to ensure it works as expected.