renew session after changing username / password

[gr2m]

account.update(options) is implemented, but the problem is that once the username and/or password changes, the session ID gets invalidated.

So what we need is to renew the session with the new username / password by doing a PUT /session request, but without triggering any extra events.

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• [x] claim this issue (@nickcolley)
• [x] update integration test at test/integration/update-account-test.js to test that account.get('session.id') and account.username has been updated
• [x] commit with test: renew session after username / password change
• [x] start a PR (https://github.com/hoodiehq/hoodie-client-account/pull/74)
• [x] implement the session update
• [x] commit with fix: renew session after username / password change & push
• [x] review & merge
• [x] update https://github.com/gr2m/milestones/issues/88
• [x] uncomment tests in app-something-tracker to make sure the session renewal works