feat(headers): add TE header struct (#1150)

The `TE` header is used by a client to specify which
transfer encodings other than `chunked` it will accept.
It also specifies whether HTTP trailers are acceptable.
This commit also adds a `Trailers` variant to the
`hyper::header::shared::Encoding` enum.

Closes #1109 

BREAKING CHANGE: The `Encoding` enum has an additional
  variant, `Trailers`.

src/header/shared/encoding.rs

@@ -1,7 +1,7 @@
 use std::fmt;
 use std::str;
 
-pub use self::Encoding::{Chunked, Brotli, Gzip, Deflate, Compress, Identity, EncodingExt};
+pub use self::Encoding::{Chunked, Brotli, Gzip, Deflate, Compress, Identity, EncodingExt, Trailers};
 
 /// A value to represent an encoding used in `Transfer-Encoding`
 /// or `Accept-Encoding` header.
@@ -19,6 +19,8 @@ pub enum Encoding {
     Compress,
     /// The `identity` encoding.
     Identity,
+    /// The `trailers` encoding.
+    Trailers,
     /// Some other encoding that is less common, can be any String.
     EncodingExt(String)
 }
@@ -32,6 +34,7 @@ impl fmt::Display for Encoding {
             Deflate => "deflate",
             Compress => "compress",
             Identity => "identity",
+            Trailers => "trailers",
             EncodingExt(ref s) => s.as_ref()
         })
     }
@@ -47,6 +50,7 @@ impl str::FromStr for Encoding {
             "gzip" => Ok(Gzip),
             "compress" => Ok(Compress),
             "identity" => Ok(Identity),
+            "trailers" => Ok(Trailers),
             _ => Ok(EncodingExt(s.to_owned()))
         }
     }