properly reset multicache

since we're allowing a nil @multi_cache object, setting it to nil not only means we don't
reload it, it means we completely ignore it and always fall back to Rails.cache

Change-Id: I9a537b00de860bb53a7cc34e605eb6ab8f6d1f6f
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/235685
Reviewed-by: James Williams <jamesw@instructure.com>
QA-Review: James Williams <jamesw@instructure.com>
Product-Review: James Williams <jamesw@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>

lib/multi_cache.rb

@@ -34,7 +34,7 @@ class MultiCache
     end
 
     def reset
-      @multi_cache = nil
+      remove_instance_variable(:@multi_cache) if instance_variable_defined?(:@multi_cache)
     end
   end