619 enable last modified header on api outputs

closes #619 
you can abstact a `last_modified_header` function to handle this work everywhere, then assign the result to `default_response_headers` here.

Thinking something like
```python
def last_modified_header(header):
    return {'Last-Modified': get_header_date_format(header_date)}
```

This makes the dict used for the modified date easily replaceable and DRYs up uses of a constant string.

you can then change this and all direct assignment lines to
```python
default_response_headers = last_modified_header(
    get_instance_date(Attachment.objects.last()))
```

and you can change line 165 and all dict assignment lines to:
```python
self.headers = merge_dicts(self.headers,
    last_modified_header(get_instance_date(self.objects)))
```

using a `merge_dicts` [helper](https://stackoverflow.com/questions/38987/how-can-i-merge-two-python-dictionaries-in-a-single-expression) function:

```python
from itertools import chain 

def merge_dicts(*args):
    return dict(chain(*[d.items() for d in args]))
```