Make Data behave more like a "struct" class, fixes T359

Test Plan:
```
% bin/phpspec run
% bin/behat -f progress -c application/tests/behat.yml
```

Reviewers: dkobia, eyedol, rjmackay

Reviewed By: rjmackay

Subscribers: srutto

Maniphest Tasks: T359

Differential Revision: https://phabricator.ushahidi.com/D184