Allow the v2 encryptor to serialize messages with `Marshal`

[jcmfernandes]

This is a follow-up on #39 that must be merged first.

I deliberately avoided allowing Marshal serialization in the v2 encryptor because 1) Marshal has been a source of RCE vulnerabilities and 2) serializing messages as JSON allows for better interoperability, making it trivial to access sessions created with rack-session in other languages.

However, it's impossible to swap Marhal with JSON without breaking users' code that relies on Marhal behavior that JSON doesn't mimic (e.g., preserving ruby symbols).

So... I see 3 ways forward:

1. We close this PR and release yet another major
2. We close this PR and make v2 opt-in instead of the default
3. We merge this PR

Happy to hear some feedback.