Oh I think this does the wrong thing. Also it should be called "getall".

It should be...

```python
h = HTTPHeaderDict()
h.append('foo', 'bar')
h.append('foo', 'quux')
h.get('foo') == 'bar, quux'
h.getall('foo') == ['bar, 'quux']
```

It should be analogous to http://docs.pylonsproject.org/projects/pyramid/en/latest/api/interfaces.html#pyramid.interfaces.IMultiDict