Add empty image alt tag if alt text not specified (#2323)

Middleman's image_tag helper wraps the Padrino image_tag helper.
By default, the Padrino image_tag helper adds an alt tag based on the
image filename when one isn't explicitly set.

Alt text based on the filename is not helpful to users and therefore
bad for accessibility.

To avoid this, explicitly set an empty alt tag value before calling
Padrino's image_tag if the user hasn't specified a value.

Co-Authored-By: vixus0 <vixus0@gmail.com>

Co-authored-by: vixus0 <vixus0@gmail.com>

middleman-core/features/default_alt_tag.feature

@@ -0,0 +1,11 @@
+Feature: Add an empty alt tag for images that don't have alt tags specified
+
+  Scenario: Image does not have alt tag specified
+    Given the Server is running at "default-alt-tags-app"
+    When I go to "/empty-alt-tag.html"
+    Then I should see 'alt=""'
+
+  Scenario: Image has alt tag specified
+    Given the Server is running at "default-alt-tags-app"
+    When I go to "/meaningful-alt-tag.html"
+    Then I should see 'alt="Meaningful alt text"'

middleman-core/features/relative_assets.feature

@@ -131,7 +131,7 @@ Feature: Relative Assets
       """
     And the Server is running at "relative-assets-app"
     When I go to "/sub/image_tag.html"
-    Then I should see '<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" />'
+    Then I should see '<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" alt="" />'
 
   Scenario: URLs are not rewritten for rewrite ignored paths
     Given a fixture app "relative-assets-app"

middleman-core/features/relative_assets_helpers_only.feature

@@ -87,7 +87,7 @@ Feature: Relative Assets (Helpers Only)
       """
     And the Server is running at "relative-assets-app"
     When I go to "/sub/image_tag.html"
-    Then I should see '<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" />'
+    Then I should see '<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" alt="" />'
 
   @wip
   Scenario: URLs are not rewritten for rewrite ignored paths

middleman-core/fixtures/default-alt-tags-app/source/empty-alt-tag.html.erb

@@ -0,0 +1 @@
+<%= image_tag "blank.gif" %>

middleman-core/fixtures/default-alt-tags-app/source/meaningful-alt-tag.html.erb

@@ -0,0 +1 @@
+<%= image_tag "blank.gif", alt: "Meaningful alt text" %>

middleman-core/lib/middleman-core/core_extensions/default_helpers.rb

@@ -282,6 +282,8 @@ def image_tag(path, params={})
         params[:srcset] = images_sources.join(', ')
       end
 
+      params[:alt] ||= ''
+
       super(path, params)
     end