refactor(header): change Header::fmt_header to take a header::Formatter

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)`.

src/header/common/retry_after.rs

@@ -121,7 +121,13 @@ impl Header for RetryAfter {
         }
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> ::std::fmt::Result {
+        f.fmt_line(self)
+    }
+}
+
+impl fmt::Display for RetryAfter {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
             RetryAfter::Delay(ref duration) => {
                 write!(f, "{}", duration.num_seconds())