Removing method before redefining it. It was causing warnings. [#5072 state:resolved]

Signed-off-by: José Valim <jose.valim@gmail.com>

activemodel/lib/active_model/observing.rb

@@ -157,6 +157,7 @@ class << self
       def observe(*models)
         models.flatten!
         models.collect! { |model| model.respond_to?(:to_sym) ? model.to_s.camelize.constantize : model }
+        remove_method(:observed_classes)
         define_method(:observed_classes) { models }
       end