feat(headers): Implementing content-encoding header

Closes #391

src/header/common/content_encoding.rs

@@ -0,0 +1,19 @@
+use header::Encoding;
+
+/// The `Content-Encoding` header.
+///
+/// This header describes the encoding of the message body. It can be
+/// comma-separated, including multiple encodings.
+///
+/// ```notrust
+/// Content-Encoding: gzip
+/// ```
+#[derive(Clone, PartialEq, Debug)]
+pub struct ContentEncoding(pub Vec<Encoding>);
+
+impl_list_header!(ContentEncoding,
+                  "Content-Encoding",
+                  Vec<Encoding>);
+
+bench_header!(single, ContentEncoding, { vec![b"gzip".to_vec()] });
+bench_header!(multiple, ContentEncoding, { vec![b"gzip, deflate".to_vec()] });