feat(header): add Encoding::Brotli variant

BREAKING CHANGE: This adds a new variant to the `Encoding` enum, which
  can break exhaustive matches.

src/header/shared/encoding.rs

@@ -1,14 +1,16 @@
 use std::fmt;
 use std::str;
 
-pub use self::Encoding::{Chunked, Gzip, Deflate, Compress, Identity, EncodingExt};
+pub use self::Encoding::{Chunked, Brotli, Gzip, Deflate, Compress, Identity, EncodingExt};
 
 /// A value to represent an encoding used in `Transfer-Encoding`
 /// or `Accept-Encoding` header.
 #[derive(Clone, PartialEq, Debug)]
 pub enum Encoding {
     /// The `chunked` encoding.
     Chunked,
+    /// The `br` encoding.
+    Brotli,
     /// The `gzip` encoding.
     Gzip,
     /// The `deflate` encoding.
@@ -25,6 +27,7 @@ impl fmt::Display for Encoding {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.write_str(match *self {
             Chunked => "chunked",
+            Brotli => "br",
             Gzip => "gzip",
             Deflate => "deflate",
             Compress => "compress",
@@ -39,6 +42,7 @@ impl str::FromStr for Encoding {
     fn from_str(s: &str) -> ::Result<Encoding> {
         match s {
             "chunked" => Ok(Chunked),
+            "br" => Ok(Brotli),
             "deflate" => Ok(Deflate),
             "gzip" => Ok(Gzip),
             "compress" => Ok(Compress),