perf(server): cache renderings of the Date header

This is actually one of the biggest impacts to benchmark performances at
this point. Caching the rendering of the Date header improves "hello
world" benchmarks by around 10%.

src/http/h1/mod.rs

@@ -1,6 +1,7 @@
 pub use self::decode::Decoder;
 pub use self::encode::Encoder;
 
+mod date;
 mod decode;
 mod encode;
 pub mod parse;