This is a breaking change. A call to `Response::headers()` can be
replaced with `Response::headers().iter()`. A call to
`Response::header_values()` can be replaced with
`Response::headers().get()`.
A few interesting notes on this breakage:
* `Cookie` how has a lifetime. It should be `'static'` everywhere.
* The `SetCookie` header is no longer reexported.
* Instead, `Cookie` implements `Into<Header>` for Set-Cookie.