fix(plugin-express): fetch request body in onError
fix(plugin-express): fetch request body in onError

This fixes an issue where the body may not exist when the request
handler runs and so the body will be missing from the error report
in the dashboard

This can happen because the body may be parsed by middleware that
runs after the requestHandler

bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js