They can be re-added later, removing now prevents that fix from being a
breaking change.
BREAKING CHANGE: The `link_extensions` methods of the `Link` header are
removed until fixed.
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)`.
