Improve the tests around merging HTML attributes

This adds a few more tests that try to ensure we're properly dealing
with custom HTML attributes.

There are a couple of small improvements to area labelledby and
describedby argument building where we wrap the id in an array to allow
the values to be cleanly merged if required.

Also add a test example with some JSON for completeness.

lib/govuk_design_system_formbuilder/elements/error_summary.rb

@@ -125,7 +125,7 @@ def options
             module: %(#{brand}-error-summary)
           },
           aria: {
-            labelledby: summary_title_id
+            labelledby: [summary_title_id.presence]
           }
         }
       end

lib/govuk_design_system_formbuilder/traits/fieldset_item.rb

@@ -24,7 +24,7 @@ def options
           id: field_id(link_errors: @link_errors),
           class: classes,
           multiple: @multiple,
-          aria: { describedby: [hint_id] },
+          aria: { describedby: [hint_id.presence] },
           data: { 'aria-controls' => @conditional_id }
         }
       end

spec/support/shared/shared_html_attribute_examples.rb

@@ -2,19 +2,55 @@
   let(:example_block) { proc { builder.tag.span('block text') } }
 
   context 'with args in the expected formats' do
+    sample_json = { e: 'f', g: %w(h i j) }.to_json
     custom_attributes = {
       required:     { provided: true,                    expected: 'required' },
       autocomplete: { provided: false,                   expected: 'false' },
       placeholder:  { provided: 'Seymour Skinner',       expected: 'Seymour Skinner' },
-      data:         { provided: { a: 'b' },              expected: { 'data-a' => 'b' } },
       class:        { provided: %w(red spots),           expected: %w(red spots) },
       value:        { provided: 'Montgomery Montgomery', expected: 'Montgomery Montgomery' },
 
-      # aria-label is a special case, along with value it should not be treated
-      # like a list when deep merging
+      data: {
+        provided: {
+          a: 'b',
+          c: 'd',
+          json: sample_json,
+        },
+        expected: {
+          'data-a'    => 'b',
+          'data-c'    => 'd',
+          'data-json' => sample_json,
+        }
+      },
       aria: {
-        provided: { c: 'd', label: 'Burns Burns' },
-        expected: { 'aria-c' => 'd', 'aria-label' => 'Burns Burns' }
+        provided: {
+          # these mergeable items all accept lists of values that are space
+          # separated strings (e.g., aria-controls: "item-a item-b") so should
+          # be deeply-merged with attributes generated by the form builder.
+          #
+          # Any duplicate values will be repeated in this proces.
+          #
+          # see: GOVUKDesignSystemFormbuilder::Traits::HTMLAttributes::Attributes::MERGEABLE)
+          controls: 'Risotto Risotto',
+          describedby: 'Wolfcastle Wolfcastle',
+          flowto: 'McClure McClure',
+          labelledby: 'Spuckler Spuckler',
+          owns: 'Muntz Muntz',
+
+          # aria-label is a special case, along with value it should not be
+          # treated like a list when deep merging
+          label: 'Burns Burns',
+
+        },
+        expected: {
+          'aria-label' => 'Burns Burns',
+
+          'aria-controls' => 'Risotto',
+          'aria-describedby' => 'Wolfcastle',
+          'aria-flowto' => 'McClure',
+          'aria-labelledby' => 'Spuckler',
+          'aria-owns' => 'Muntz',
+        }
       },
     }
 
@@ -53,7 +89,7 @@
         when key.in?(special_keys)
           specify "#{val} is set" do
             val.each do |data_attribute_name, data_attribute_value|
-              expect(subject).to have_tag(described_element, with: { data_attribute_name => data_attribute_value })
+              expect(parsed_subject.at_css(described_element)[data_attribute_name]).to match(data_attribute_value)
             end
           end