feat(http): add optional serialization of common types via serde

This is behind a Cargo feature to avoid forcing downstream users to
depend on `serde`. It is needed for Servo IPC to work.

src/header/common/mod.rs

@@ -158,6 +158,31 @@ macro_rules! test_header {
     }
 }
 
+#[macro_export]
+macro_rules! __hyper_generate_header_serialization {
+    ($id:ident) => {
+        #[cfg(feature = "serde-serialization")]
+        impl ::serde::Serialize for $id {
+            fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error>
+                            where S: ::serde::Serializer {
+                format!("{}", self).serialize(serializer)
+            }
+        }
+
+        #[cfg(feature = "serde-serialization")]
+        impl ::serde::Deserialize for $id {
+            fn deserialize<D>(deserializer: &mut D) -> Result<$id, D::Error>
+                              where D: ::serde::Deserializer {
+                let string_representation: String =
+                    try!(::serde::Deserialize::deserialize(deserializer));
+                Ok($crate::header::Header::parse_header(&[
+                    string_representation.into_bytes()
+                ]).unwrap())
+            }
+        }
+    }
+}
+
 #[macro_export]
 macro_rules! header {
     // $a:meta: Attributes associated with the header item (usually docs)
@@ -190,6 +215,8 @@ macro_rules! header {
                 self.fmt_header(f)
             }
         }
+
+        __hyper_generate_header_serialization!($id);
     };
     // List header, one or more items
     ($(#[$a:meta])*($id:ident, $n:expr) => ($item:ty)+) => {
@@ -216,6 +243,7 @@ macro_rules! header {
                 self.fmt_header(f)
             }
         }
+        __hyper_generate_header_serialization!($id);
     };
     // Single value header
     ($(#[$a:meta])*($id:ident, $n:expr) => [$value:ty]) => {
@@ -241,6 +269,7 @@ macro_rules! header {
                 ::std::fmt::Display::fmt(&**self, f)
             }
         }
+        __hyper_generate_header_serialization!($id);
     };
     // List header, one or more items with "*" option
     ($(#[$a:meta])*($id:ident, $n:expr) => {Any / ($item:ty)+}) => {
@@ -281,6 +310,7 @@ macro_rules! header {
                 self.fmt_header(f)
             }
         }
+        __hyper_generate_header_serialization!($id);
     };
 
     // optional test module