Add reify_original method.

Returning the original version is a common use-case and is not as easy as it should be, particularly because calling `reify` on the first version returns `nil`, which is the `object` *before* it was created.

This method simply tries to `reify` from the *second* version, if it exists. If it doesn’t exist (i.e. there has not been any changes to the object), it falls back to `self`, which is the original and *only* version of the object.

Fixes paper-trail-gem#1204.

lib/paper_trail/record_trail.rb

@@ -69,6 +69,13 @@ def record_create
       end
     end
 
+    # Returns the original version of this object or just this object if there has been no changes.
+    #
+    # @api public
+    def reify_original
+      versions.second&.reify || @record
+    end
+
     # `recording_order` is "after" or "before". See ModelConfig#on_destroy.
     #
     # @api private