Make ActionDispatch::Request::Session#fetch behave like Hash#fetch

Session#fetch was mutating the session when given a default argument
and/or a block. Since Session duck-types as a Hash, it should behave
like one in these cases.
Update CHANGELOG entry for Session#fetch

This reverts the changes to CHANGELOG.md in commit
38f8872aa5fd8f0a1d0895e9eb41f73261acd040.
Merge pull request #13279 from misfo/session-fetch-that-behaves-like-hash-fetch

Make ActionDispatch::Request::Session#fetch behave like Hash#fetch