Fixing set_initial_state with Mongoid

ActiveModel::Transitions#set_initial_state was not working with mongoid.
It was check for has_attribute?(:state) which returns false if the field
has no value in a mongoid document.

Using respond_to?(:state=) fixes the issue and keeps the behavior.

lib/active_model/transitions.rb

@@ -63,7 +63,7 @@ def read_state
     end
 
     def set_initial_state
-      self.state ||= self.class.get_state_machine.initial_state.to_s if self.has_attribute?(:state)
+      self.state ||= self.class.get_state_machine.initial_state.to_s if self.respond_to?(:state=)
     end
 
     def state_inclusion