feat(header): add prefer and preference applied headers

Closes #747

src/header/common/preference_applied.rs

@@ -0,0 +1,100 @@
+use std::fmt;
+use header::{Header, HeaderFormat, Preference};
+use header::parsing::{from_comma_delimited, fmt_comma_delimited};
+
+/// `Preference-Applied` header, defined in [RFC7240](http://tools.ietf.org/html/rfc7240)
+///
+/// The `Preference-Applied` response header may be included within a
+/// response message as an indication as to which `Prefer` header tokens were
+/// honored by the server and applied to the processing of a request.
+///
+/// # ABNF
+/// ```plain
+/// Preference-Applied = "Preference-Applied" ":" 1#applied-pref
+/// applied-pref = token [ BWS "=" BWS word ]
+/// ```
+///
+/// # Example values
+/// * `respond-async`
+/// * `return=minimal`
+/// * `wait=30`
+///
+/// # Examples
+/// ```
+/// use hyper::header::{Headers, PreferenceApplied, Preference};
+///
+/// let mut headers = Headers::new();
+/// headers.set(
+///     PreferenceApplied(vec![Preference::RespondAsync])
+/// );
+/// ```
+/// ```
+/// use hyper::header::{Headers, PreferenceApplied, Preference};
+///
+/// let mut headers = Headers::new();
+/// headers.set(
+///     PreferenceApplied(vec![
+///         Preference::RespondAsync,
+///         Preference::ReturnRepresentation,
+///         Preference::Wait(10u32),
+///         Preference::Extension("foo".to_owned(),
+///                               "bar".to_owned(),
+///                               vec![]),
+///     ])
+/// );
+/// ```
+#[derive(PartialEq, Clone, Debug)]
+pub struct PreferenceApplied(pub Vec<Preference>);
+
+__hyper__deref!(PreferenceApplied => Vec<Preference>);
+
+impl Header for PreferenceApplied {
+    fn header_name() -> &'static str {
+        "Preference-Applied"
+    }
+
+    fn parse_header(raw: &[Vec<u8>]) -> ::Result<PreferenceApplied> {
+        let preferences = try!(from_comma_delimited(raw));
+        if !preferences.is_empty() {
+            Ok(PreferenceApplied(preferences))
+        } else {
+            Err(::Error::Header)
+        }
+    }
+}
+
+impl HeaderFormat for PreferenceApplied {
+    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let preferences: Vec<_> = self.0.iter().map(|pref| match pref {
+            // The spec ignores parameters in `Preferences-Applied`
+            &Preference::Extension(ref name, ref value, _) => Preference::Extension(
+              name.to_owned(),
+              value.to_owned(),
+              vec![]
+            ),
+            preference @ _ => preference.clone()
+        }).collect();
+        fmt_comma_delimited(f, &preferences)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use header::{HeaderFormat, Preference};
+    use super::*;
+
+    #[test]
+    fn test_format_ignore_parameters() {
+        assert_eq!(
+            format!("{}", &PreferenceApplied(vec![Preference::Extension(
+                "foo".to_owned(),
+                "bar".to_owned(),
+                vec![("bar".to_owned(), "foo".to_owned()), ("buz".to_owned(), "".to_owned())]
+            )]) as &(HeaderFormat + Send + Sync)),
+            "foo=bar".to_owned()
+        );
+    }
+}
+
+bench_header!(normal,
+    PreferenceApplied, { vec![b"respond-async, return=representation".to_vec(), b"wait=100".to_vec()] });