Fix HTTP Accept header for list query

Given following list function

function(head, req) {
  provides('text', function(){
    start({
      'headers': {
        'Content-Type': 'text/plain'
      }
    });
    send('Hello list!');
  });
}

It will get HTTP 406 error, because the default Accept header is set to application/json.

See Also: couchapp/couchapp#234