Make ExcerptHelper work for String inputs

app/helpers/excerpt_helper.rb

@@ -2,9 +2,9 @@ module ExcerptHelper
   def format_excerpt(content, length: 200)
     return "" if content.blank?
 
-    text = content.to_plain_text
+    text = content.respond_to?(:to_plain_text) ? content.to_plain_text : content.to_s
     text = text.gsub(/^>\s*(.*)$/m, '> \1')
-    text = text.gsub(/^[-*]\s*(.*)$/m, '• \1')
+    text = text.gsub(/^\s*[-+]\s*(.*)$/m, '• \1')
     text = text.gsub(/^\d+\.\s*(.*)$/m) { |m| m }
     text = text.gsub(/\s+/, " ").strip
     text.truncate(length)

test/helpers/excerpt_helper_test.rb

@@ -26,5 +26,6 @@ class ExcerptHelperTest < ActionView::TestCase
   private
     def assert_excerpt(expected, content, ...)
       assert_equal expected, format_excerpt(ActionText::Content.new(content), ...), "Excerpt of Action Text Content does not match"
+      assert_equal expected, format_excerpt(content, ...), "Excerpt of String does not match"
     end
 end