Motivation:
`Date`, `Expires`, and `Set-Cookie` headers are being generated with a 1-digit day of month,
e.g. `Sun, 6 Nov 1994 08:49:37 GMT`. RFC 2616 specifies that `Date` and `Expires` headers should
use "a fixed-length subset of that defined by RFC 1123" which includes a 2-digit day of month.
RFC6265 is lax in it's specification of the `Set-Cookie` header and permits a 2-digit day of month.
See: https://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html
See: https://tools.ietf.org/html/rfc1123#page-55
See: https://tools.ietf.org/html/rfc6265#section-5.1.1
Modifications:
- Update `DateFormatter` to correctly implement RFC 2616 headers
Result:
```
Date: Sun, 06 Nov 1994 08:49:37 GMT
Expires: Sun, 06 Nov 1994 08:49:37 GMT
Set-Cookie: id=a3fWa; Expires=Sun, 06 Nov 1994 08:49:37 GMT
```
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