feat($http): accept function as headers value

So we can request with dynamic header value

module.factory('Res', [
  '$resource'
  '$routeParams'
  'globalConfig'

function($resource, $routeParams, globalConfig) {
  resource(
    '/url/:id',
    {id: function() { return $routeParams.id; }},
    {patch: method: 'patch', headers: {'X-Token': function() { return globalConfig.token; }}}
  )
}]);