Explicitly check that values for notifying are strings.

Some background: we have a custom session adaptor set to $_SESSION that implements ArrayAccess. When trying out bugsnag we kept getting errors in our error handling and couldn't submit anything.

It appears that because transform always checkes mb_detect_encoding() that it's safe to assume all values are meant to be strings, so this should be a sane short term fix.

Longer term: we'd like to be able to populate the session tab, is there a change to the API where we can maybe set a function for populating session manually?