feat($http): accept function as headers value

So we can request with dynamic header value.

```javascript
module.factory('Res', [
  '$resource'
  '$routeParams'
  'globalConfig'

function($resource, $routeParams, globalConfig) {
  resource('/url/:id', {id: "@id"}, {
    patch: {
      method: 'patch',
      headers: {
        'Authorization': function() {
          return "token " + globalConfig.token;
        }
      }
    }
  });
}]);
```