Fix an uninitialized variable warning in RotationCoordinator

Author: byroot

activesupport/lib/active_support/messages/rotation_coordinator.rb

@@ -9,6 +9,7 @@ def initialize(&secret_generator)
         raise ArgumentError, "A secret generator block is required" unless secret_generator
         @secret_generator = secret_generator
         @rotate_options = []
+        @on_rotation = nil
         @codecs = {}
       end