Random.new returns differently seeded objects

Specs of [`Random#bytes`][0] suggest that instances of `Random` returned by `Random.new` should be seeded differently. Specs of `Random.new` did not specify this behaviour. This commit introduces an appropriate test.

[0]: https://github.com/rubyspec/rubyspec/blob/c1ecc82/core/random/bytes_spec.rb#L37