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/lib.rs

@@ -133,6 +133,8 @@ extern crate time;
 extern crate url;
 #[cfg(feature = "openssl")]
 extern crate openssl;
+#[cfg(feature = "serde-serialization")]
+extern crate serde;
 extern crate cookie;
 extern crate unicase;
 extern crate httparse;