Fix frozen string error

Author: nygrenh

app/helpers/application_helper.rb

@@ -76,7 +76,7 @@ def bs_labeled_field(label, field, options = {})
     str = ''
     case type
     when :boolean
-      str = '<label class="checkbox">'
+      str = '<label class="checkbox">'.dup
       str << "  #{field}#{label}"
       str << '</label>'
     else