Adds .to_s to table_name call

Avoids a NoMethodError when table_name is a symbol instead of a string.

activerecord/lib/active_record/table_metadata.rb

@@ -44,7 +44,7 @@ def associated_with?(association_name)
     end
 
     def associated_table(table_name)
-      association = klass._reflect_on_association(table_name) || klass._reflect_on_association(table_name.singularize)
+      association = klass._reflect_on_association(table_name) || klass._reflect_on_association(table_name.to_s.singularize)
 
       if !association && table_name == arel_table.name
         return self