Check that content-type starts with application/json

When Chrome sends the AJAX request to add a user to the course team, it sets the
Content-type to "application/json". However, when Firefox sends the same request,
it sets the Content-type to "application/json; charset=UTF-8". This commit only
checks that the Content-type begins with "application/json", not is identical
to it; that way, Firefox can play, too.