Fix string substitution regression

Author: nicoco007

lib/rails/dom/testing/assertions/selector_assertions/substitution_context.rb

@@ -24,7 +24,7 @@ def match(matches, attribute, matcher)
             private
               def matcher_for(value, format_for_presentation)
                 # Nokogiri doesn't like arbitrary values without quotes, hence inspect.
-                if format_for_presentation
+                if format_for_presentation || value.is_a?(String)
                   value.inspect # Avoid to_s so Regexps aren't put in quotes.
                 else
                   "\"#{value}\""

test/selector_assertions_test.rb

@@ -136,6 +136,11 @@ def test_substitution_values
     end
   end
 
+  def test_substitution_value_with_quotes
+    render_html '<input placeholder="placeholder with &quot;quotes&quot;" />'
+    assert_select "input[placeholder=?]", 'placeholder with "quotes"'
+  end
+
   def test_multiple_substitution_values
     render_html '<input name="foo[12]" value="34">'
     assert_select ":match('name', ?):match('value', ?)", /\w+\[\d+\]/, /\d+/