Use reprs of values in _CacheControl.

Prevents things like:

```
exceptions.AssertionError: <ResponseCacheControl 'no-cache=True, no-store=True, private=True, max-age=0'> != <ResponseCacheControl 'no-cache=True, no-store=True, private=True, max-age=0'>
```

caused by using `to_header` in there.