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/common/te.rs

@@ -0,0 +1,67 @@
+use header::{Encoding, QualityItem};
+
+header! {
+    /// `TE` header, defined in
+    /// [RFC7230](http://tools.ietf.org/html/rfc7230#section-4.3)
+    ///
+    /// As RFC7230 states, "The "TE" header field in a request indicates what transfer codings,
+    /// besides chunked, the client is willing to accept in response, and
+    /// whether or not the client is willing to accept trailer fields in a
+    /// chunked transfer coding."
+    ///
+    /// For HTTP/1.1 compliant clients `chunked` transfer codings are assumed to be acceptable and
+    /// so should never appear in this header.
+    ///
+    /// # ABNF
+    /// ```plain
+    /// TE        = "TE" ":" #( t-codings )
+    /// t-codings = "trailers" | ( transfer-extension [ accept-params ] )
+    /// ```
+    ///
+    /// # Example values
+    /// * `trailers`
+    /// * `trailers, deflate;q=0.5`
+    /// * ``
+    ///
+    /// # Examples
+    /// ```
+    /// use hyper::header::{Headers, TE, Encoding, qitem};
+    ///
+    /// let mut headers = Headers::new();
+    /// headers.set(
+    ///     TE(vec![qitem(Encoding::Trailers)])
+    /// );
+    /// ```
+    /// ```
+    /// use hyper::header::{Headers, TE, Encoding, qitem};
+    ///
+    /// let mut headers = Headers::new();
+    /// headers.set(
+    ///     TE(vec![
+    ///         qitem(Encoding::Trailers),
+    ///         qitem(Encoding::Gzip),
+    ///         qitem(Encoding::Deflate),
+    ///     ])
+    /// );
+    /// ```
+    /// ```
+    /// use hyper::header::{Headers, TE, Encoding, QualityItem, Quality, qitem};
+    ///
+    /// let mut headers = Headers::new();
+    /// headers.set(
+    ///     TE(vec![
+    ///         qitem(Encoding::Trailers),
+    ///         QualityItem::new(Encoding::Gzip, Quality(600)),
+    ///         QualityItem::new(Encoding::EncodingExt("*".to_owned()), Quality(0)),
+    ///     ])
+    /// );
+    /// ```
+    (TE, "TE") => (QualityItem<Encoding>)*
+
+    test_te {
+        // From the RFC
+        test_header!(test1, vec![b"trailers"]);
+        test_header!(test2, vec![b"trailers, deflate;q=0.5"]);
+        test_header!(test3, vec![b""]);
+    }
+}