fix(header): enable SetCookie.fmt_header when only 1 cookie

src/header/common/set_cookie.rs

@@ -107,8 +107,12 @@ impl Header for SetCookie {
 }
 
 impl HeaderFormat for SetCookie {
-    fn fmt_header(&self, _f: &mut fmt::Formatter) -> fmt::Result {
-        panic!("SetCookie cannot be used with fmt_header, must use fmt_multi_header");
+    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        if self.0.len() == 1 {
+            write!(f, "{}", &self.0[0])
+        } else {
+            panic!("SetCookie with multiple cookies cannot be used with fmt_header, must use fmt_multi_header");
+        }
     }
 
     fn fmt_multi_header(&self, f: &mut ::header::MultilineFormatter) -> fmt::Result {

src/header/mod.rs

@@ -539,28 +539,32 @@ impl<'a> FromIterator<HeaderView<'a>> for Headers {
 impl<'a> fmt::Display for &'a (HeaderFormat + Send + Sync) {
     #[inline]
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        (**self).fmt_header(f)
+        let mut multi = MultilineFormatter(Multi::Join(true, f));
+        self.fmt_multi_header(&mut multi)
     }
 }
 
 /// A wrapper around any Header with a Display impl that calls fmt_header.
 ///
 /// This can be used like so: `format!("{}", HeaderFormatter(&header))` to
-/// get the representation of a Header which will be written to an
-/// outgoing `TcpStream`.
+/// get the 'value string' representation of this Header.
+///
+/// Note: This may not necessarily be the value written to stream, such
+/// as with the SetCookie header.
 pub struct HeaderFormatter<'a, H: HeaderFormat>(pub &'a H);
 
 impl<'a, H: HeaderFormat> fmt::Display for HeaderFormatter<'a, H> {
     #[inline]
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.0.fmt_header(f)
+        let mut multi = MultilineFormatter(Multi::Join(true, f));
+        self.0.fmt_multi_header(&mut multi)
     }
 }
 
 impl<'a, H: HeaderFormat> fmt::Debug for HeaderFormatter<'a, H> {
     #[inline]
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.0.fmt_header(f)
+        fmt::Display::fmt(self, f)
     }
 }