Recommended usage or defaults for stringify / confusing default behavior

[oldshaghat]

• I'm submitting a ...
  [x] bug report
  [ ] feature request
  [ ] question about the decisions made in the repository
  [x] question about how to use this project

• Summary
  Probably a user error not a bug but - I was migrating to this from connect-mongodb-session; I have passport 0.7.0, express 4.19.2, express-session 1.18.0. Under a basic config that appeared to match the suggestions I was seeing an Object.parse error from MongoStore.js:220 ; const s = session && this.transformFunctions.unserialize(session.session);

This was fixed by supplying 'stringify : false' in my config options; I am not sure I understand what is happening under the hood but maybe this could be added to the readme if this is all expected behavior.