Gem can generate invalid duplicate data attributes on buttons

[DavidBiddle]

In some cases, the gem can generate two identical data attributes on a single button.

Steps to reproduce

Create a button with a data-module attribute using the Rails nested hash convention:

<%= govuk_button_to 'A button', '#', data: { module: "special-js-button" } %>

observe that the generated HTML now contains two data-module atttributes:

<button class="govuk-button" data-module="govuk-button" new_tab="false" data-module="special-js-button" type="submit">A button</button>

When this happens, browsers will discard the second data-module value as it is invalid.

Note that if you use the full data-module attribute instead:

<%= govuk_button_to 'A button', '#', 'data-module': "special-js-button" } %>

the generated HTML only contains our custom data-module atttribute:

<button class="govuk-button" data-module="special-js-button" new_tab="false" type="submit">A button</button>

Expected behaviour

I would expect to be able to overwrite the data-module attribute using the nested hash convention.

Suspected cause

I think this is happening because of the way build_data_attributes works - it uses the full data-module attribute rather than the nested hash:

def build_data_attributes(data_module, prevent_double_click: nil)
    {
      "data-module": data_module,
      "data-prevent-double-click": prevent_double_click
    }.compact
  end

The govuk_button_to helper uses deep_merge_html_attributes which comes from the x-govuk/html-attributes-utils gem. Issue 1 in that repo says:

This library, and both the components and form builder assume people are using the latter. Using the former might lead to unintended consequences.

So I think the solution here might be to use the nested hash convention in build_data_attributes:

def build_data_attributes(data_module, prevent_double_click: nil)
    {
        data: {
          module: data_module,
          'prevent-double-click': prevent_double_click
        }.compact
    }
end