The accept header is poorly implemented by browsers and causes strange errors when used on public sites where crawlers make requests too.  You should use formatted urls (e.g. /people/1.xml) to support API clients. Alternatively to re-enable it you need to set:

config.action_controller.use_accept_header = true

A special case remains for ajax requests which will have a javascript format for the base resource (/people/1) if the X-Requested-With header is present.  This lets ajax pages still use format.js despite there being no params[:format]