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

src/header/common/set_cookie.rs

@@ -90,8 +90,13 @@ impl Header for SetCookie {
             Err(::Error::Header)
         }
     }
-    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");
+        }
     }