Invoke original inherited callback when subclassed

When AASM is included into a class, it defines inherited, clobbering
the original definition.  This problem becomes readily apparent with STI
in Rails, where the original method is part of the implementation of
inheritable attributes, a feature relied on for several ActiveRecord
features such as callbacks and timestamp recording.

Move the implementation of inherited to ClassMethods and invoke the
original method via super.

lib/aasm.rb

@@ -18,15 +18,14 @@ module AASM
     base.extend AASM::ClassMethods
     AASM::Persistence.set_persistence(base)
     AASM::StateMachine[base] = AASM::StateMachine.new('')
-
-    base.class_eval do
-      def base.inherited(klass)
-        AASM::StateMachine[klass] = AASM::StateMachine[self].dup
-      end
-    end
   end
 
   module ClassMethods
+    def inherited(klass)
+      AASM::StateMachine[klass] = AASM::StateMachine[self].dup
+      super
+    end
+
     def aasm_initial_state(set_state=nil)
       if set_state
         AASM::StateMachine[self].initial_state = set_state

spec/unit/aasm_spec.rb

@@ -70,6 +70,16 @@ describe AASM, '- class level definitions' do
 end
 
 
+describe AASM, '- when included' do
+  it 'should invoke the original inherited callback when subclassed' do
+    parent = Class.new
+    parent.should_receive(:inherited)
+    parent.send(:include, AASM)
+    child = Class.new(parent)
+  end
+end
+
+
 describe AASM, '- aasm_states_for_select' do
   it "should return a select friendly array of states in the form of [['Friendly name', 'state_name']]" do
     Foo.aasm_states_for_select.should == [['Open', 'open'], ['Closed', 'closed']]