fix(session): fix double serialisation of values when inserting data into session state

[zignis]

PR Type

Bug Fix

PR Checklist

• [x] Tests for the changes have been added / updated.
• [x] Documentation comments have been added / updated.
• [x] A changelog entry has been made for the appropriate packages.
• [x] Format code with the nightly rustfmt (cargo +nightly fmt).

Overview

Replaces the usage of HashMap in session with serde Map, which solves the issue of double-serialization of values when inserting data into the session state.

Closes #308

[0rangeFox]

Hey 👋. I'm experiencing this issue, can I participate on this PR to be merged asap? At this moment I have a messy workaround and this would simplify the logic and clear my messy code just for this.