`Module#const_get` was unreliable on returning the right constant. We should…

`Module#const_get` was unreliable on returning the right constant. We should check if the constant has been defined first before trying to call `Module#const_get`

lib/paperclip.rb

@@ -141,11 +141,11 @@ module Paperclip
       # https://github.com/rails/rails/blob/v3.0.9/activesupport/lib/active_support/inflector/methods.rb#L89
       if Module.method(:const_get).arity == 1
         class_name.split('::').inject(Object) do |klass, partial_class_name|
-          klass.const_get(partial_class_name)
+          klass.const_defined?(partial_class_name) ? klass.const_get(partial_class_name) : klass.const_missing(partial_class_name)
         end
       else
         class_name.split('::').inject(Object) do |klass, partial_class_name|
-          klass.const_get(partial_class_name, false)
+          klass.const_defined?(partial_class_name) ? klass.const_get(partial_class_name, false) : klass.const_missing(partial_class_name)
         end
       end
     rescue ArgumentError => e