session#fetch doesn't behave exactly like Hash#fetch.

Mention it in the changelog and add a test checking for regressions.

Hash#fetch isn't adding the defaultly returned value.
However, in the session, saving it is the behavior we should expect.

See discussion in #12692
Merge pull request #12702 from dmathieu/finetune_session_fetch

session#fetch doesn't behave exactly like Hash#fetch.