This removes the need for someone to use the `time` crate to create a
date compatible with HTTP headers. It now works with the `SystemTime`
type from the standard library.
BREAKING CHANGE: `HttpDate` no longer has public fields. Convert between
`HttpDate` and `SystemTime` as needed.
The `header::Formatter` ensures that a formatted header is written to a
line, and allows for headers that require multiple lines. The only
header to specifically require this is `Set-Cookie`.
BREAKING CHANGE: The `fmt_header` method has changed to take a different
formatter. In most cases, if your header also implements
`fmt::Display`, you can just call `f.fmt_line(self)`.
