When a job was just deserialized `arguments` is `nil` and the serialized
arguments are in the `@serialized_arguments` variable. If we try to
serialize this job again the arguments are going to be `nil` instead of
what was serialized.

The test we had was not checking this case because it was deserializing
the job in the same object that had the arguments.

To fix this, when the `@serialized_arguments` are present we return it
instead of the result of the `arguments` serialized.