[ci skip] collection_radio_buttons options example

[andre-lgf]

Added example of how to access an option attibute passed to builder in case the person wants to add a custom style to a programatically checked value e.g.

Summary

Added example of how to access option passed to the builder (collection_check_boxes).

Other Information

Spent some time figuring out how to add a custom class to a default checked value, since I couldn't find it anywhere in the guide I decided to add it to save others some trouble.

In my case, I was creating a form and wanted to set the first value checked and also add the active class for styling (I'm using bootstrap 4).
Source of code:

<div class='col-md-4 col-sm-12 my-auto mx-auto'>
	<p><%= t('Currency') %></p>
	<div class="btn-group btn-group-toggle" data-toggle="buttons">
		<%= collection_radio_buttons :pricing, :currency_id, @currencies, :id, :symbol, {checked: @currencies.first.id, required: true} do |rb| %>
			<%= rb.label(class: "btn btn-secondary #{'active' if rb.instance_values['input_html_options'][:checked]}") { rb.radio_button + rb.text } %>
		<% end %>
	</div>
</div>

[rafaelfranca]

input_html_options is not public API and people should not be using in they applications. Can you remove this example since it can't be implemented using public APIs?

[rafaelfranca]

You could do 'my-custom-class' if rb.value == Author.last to use public API

[andre-lgf]

Actually this is way better, thank you. Must be Author.last.id (or any other value used as key) to work though. Thank you for the response, please ignore this PR.

[andre-lgf]

Updated with the correct example.