Fix healthcheck endpoint with no 'Accept:' header.

The Nagios check hits this endpoint, and expects a 200 response to
consider the application up.  This was previously returning a 406
because the nagios request didn't include an 'Accept:' header.

Change this to always return json regardless, which makes it more robust
to this sort of issue.
Merge pull request #242 from alphagov/fix_healthcheck

Fix healthcheck endpoint with no 'Accept:' header.