feat(headers): Add Pragma header field

Add the HTTP/1.0 `Pragma` header field used to prevent older Caches, that
do not understand the `Cache-Control` header field from caching the ressource.

Closes #237

src/header/common/pragma.rs

@@ -0,0 +1,61 @@
+use std::fmt;
+use std::ascii::AsciiExt;
+
+use header::{Header, HeaderFormat, parsing};
+
+/// The `Pragma` header defined by HTTP/1.0.
+///
+/// > The "Pragma" header field allows backwards compatibility with
+/// > HTTP/1.0 caches, so that clients can specify a "no-cache" request
+/// > that they will understand (as Cache-Control was not defined until
+/// > HTTP/1.1).  When the Cache-Control header field is also present and
+/// > understood in a request, Pragma is ignored.
+
+/// > In HTTP/1.0, Pragma was defined as an extensible field for
+/// > implementation-specified directives for recipients.  This
+/// > specification deprecates such extensions to improve interoperability.
+///
+/// Spec: https://tools.ietf.org/html/rfc7234#section-5.4
+#[derive(Clone, PartialEq, Debug)]
+pub enum Pragma {
+    /// Corresponds to the `no-cache` value.
+    NoCache,
+    /// Every value other than `no-cache`.
+    Ext(String),
+}
+
+impl Header for Pragma {
+    fn header_name() -> &'static str {
+        "Pragma"
+    }
+
+    fn parse_header(raw: &[Vec<u8>]) -> Option<Pragma> {
+        parsing::from_one_raw_str(raw).and_then(|s: String| {
+            let slice = &s.to_ascii_lowercase()[];
+            match slice {
+                "" => None,
+                "no-cache" => Some(Pragma::NoCache),
+                _ => Some(Pragma::Ext(s)),
+            }
+        })
+    }
+}
+
+impl HeaderFormat for Pragma {
+    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            Pragma::NoCache => write!(f, "no-cache"),
+            Pragma::Ext(ref string) => write!(f, "{}", string),
+        }
+    }
+}
+
+#[test]
+fn test_parse_header() {
+    let a: Pragma = Header::parse_header([b"no-cache".to_vec()].as_slice()).unwrap();
+    let b = Pragma::NoCache;
+    assert_eq!(a, b);
+    let c: Pragma = Header::parse_header([b"FoObar".to_vec()].as_slice()).unwrap();
+    let d = Pragma::Ext("FoObar".to_string());
+    assert_eq!(c, d);
+}