Serialize the `last_request_at` entry as an Integer

Pushing the `Time` object inside the session has inconsistencies
across different serializers and we should use a more primitive type
so we don't need any specific parsing logic for the JSON serializer.
Merge pull request #2954 from plataformatec/lm-last-request-at-json-serialization

Serialize the `last_request_at` entry as an Integer