FIX: Cache keys should be strings (#25791)

* FIX: Cache keys should be strings

Otherwise, there are subtle bugs that don't show up with a single
process.

lib/distributed_cache.rb

@@ -14,6 +14,8 @@ class DistributedCache < MessageBus::DistributedCache
   end
 
   def defer_get_set(k, &block)
+    raise TypeError if !Rails.env.production? && !k.is_a?(String)
+
     return self[k] if hash.key? k
     value = block.call
     self.defer_set(k, value)