Fix have_and_belong_to_many assertion in Rails 4

NEWS.md

@@ -1,5 +1,7 @@
 # HEAD
 
+* Fix `have_and_belong_to_many` matcher issue for Rails 4.
+
 # v 2.1.0
 
 * Add missing `failure_message_for_should_not` implementations to

lib/shoulda/matchers/active_record/association_matcher.rb

@@ -309,7 +309,11 @@ module Shoulda # :nodoc:
         end
 
         def join_table
-          reflection.options[:join_table].to_s
+          if reflection.respond_to? :join_table
+            reflection.join_table.to_s
+          else
+            reflection.options[:join_table].to_s
+          end
         end
 
         def associated_class