When caching, return the object's cache_key up front if it's defined.

This will prevent objects PORO objects that don't have updated_at defined, from throwing an error.

Not as big a deal now that PORO objects can inherit ActiveModelSerializers::Model, but still necessary if it's not inherited for whatever reason.

Add the Adapter type to the cache key.

This prevents incorrect results when the same object is serialized with different adapters.

BF:

Cherry-pick of
040a97b9e9
which was a squash of
f89ed71058

from pr 1346

test/fixtures/poro.rb

@@ -55,8 +55,8 @@ Bio      = Class.new(Model)
 Blog     = Class.new(Model) do
   FILE_DIGEST = Digest::MD5.hexdigest(File.open(__FILE__).read)
 
-  def cache_key_with_digest
-    "#{cache_key}/#{FILE_DIGEST}"
+  def digest
+    FILE_DIGEST
   end
 end
 Role     = Class.new(Model)