[Haml] Make sure the :data attribute is merged properly when using attribute methods.

lib/haml/buffer.rb

@@ -233,10 +233,11 @@ RUBY
 
     # Merges two attribute hashes.
     # This is the same as `to.merge!(from)`,
-    # except that it merges id and class attributes.
+    # except that it merges id, class, and data attributes.
     #
     # ids are concatenated with `"_"`,
     # and classes are concatenated with `" "`.
+    # data hashes are simply merged.
     #
     # Destructively modifies both `to` and `from`.
     #
@@ -259,6 +260,16 @@ RUBY
         from['class'] ||= to['class']
       end
 
+      from_data = from['data'].is_a?(Hash)
+      to_data = to['data'].is_a?(Hash)
+      if from_data && to_data
+        to['data'] = to['data'].merge(from['data'])
+      elsif to_data
+        to = Haml::Util.map_keys(to.delete('data')) {|name| "data-#{name}"}.merge(to)
+      elsif from_data
+        from = Haml::Util.map_keys(from.delete('data')) {|name| "data-#{name}"}.merge(from)
+      end
+
       to.merge!(from)
     end
 

test/haml/engine_test.rb

@@ -1214,6 +1214,27 @@ SASS
       render("%div{'data-foo' => 'first', :data => {:foo => 'second'}}"))
   end
 
+  def test_html5_data_attributes_with_attr_method
+    Haml::Helpers.module_eval do
+      def data_hash
+        {:data => {:foo => "bar", :baz => "bang"}}
+      end
+
+      def data_val
+        {:data => "dat"}
+      end
+    end
+
+    assert_equal("<div data-baz='bang' data-brat='wurst' data-foo='blip'></div>\n",
+      render("%div{data_hash, :data => {:foo => 'blip', :brat => 'wurst'}}"))
+    assert_equal("<div data-baz='bang' data-foo='blip'></div>\n",
+      render("%div{data_hash, 'data-foo' => 'blip'}"))
+    assert_equal("<div data-baz='bang' data-foo='bar' data='dat'></div>\n",
+      render("%div{data_hash, :data => 'dat'}"))
+    assert_equal("<div data-brat='wurst' data-foo='blip' data='dat'></div>\n",
+      render("%div{data_val, :data => {:foo => 'blip', :brat => 'wurst'}}"))
+  end
+
   # New attributes
 
   def test_basic_new_attributes