refactor(header): Implement HttpDate, a wrapper for dates

Using `time::Tm` directly in HTTP header fields requires special handling to parse and format
the header values., this stops us from using the header macros. By wrapping `time::Time` in a
`HttpDate`, we can use the `FromStr` and `Display` traits of `HttpDate` like for most other values.

BREAKING_CHANGE: All code using one of the `Date`, `Expires`, `If-Modified-Since`,
`If-Unmodified-Since`, `Last-Modified` header fields needs to wrap `time::Tm`
with `HttpDate`. Removed `FromStr` trait of `Date`, `If-Modified-Sice` and `If-Unmodified-Sice`,
implementing the trait here is inconsistent with other headers.

src/header/common/date.rs

@@ -1,45 +1,10 @@
-use std::fmt;
-use std::str::FromStr;
-use time::Tm;
-use header::{Header, HeaderFormat};
-use header::parsing::from_one_raw_str;
-use header::parsing::tm_from_str;
+use header::HttpDate;
 
-// Egh, replace as soon as something better than time::Tm exists.
 /// The `Date` header field.
 #[derive(Copy, PartialEq, Clone, Debug)]
-pub struct Date(pub Tm);
+pub struct Date(pub HttpDate);
 
-deref!(Date => Tm);
-
-impl Header for Date {
-    fn header_name() -> &'static str {
-        "Date"
-    }
-
-    fn parse_header(raw: &[Vec<u8>]) -> Option<Date> {
-        from_one_raw_str(raw)
-    }
-}
-
-
-impl HeaderFormat for Date {
-    fn fmt_header(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        let tm = self.0;
-        let tm = match tm.tm_utcoff {
-            0 => tm,
-            _ => tm.to_utc(),
-        };
-        fmt::Display::fmt(&tm.rfc822(), fmt)
-    }
-}
-
-impl FromStr for Date {
-    type Err = ();
-    fn from_str(s: &str) -> Result<Date, ()> {
-        tm_from_str(s).map(Date).ok_or(())
-    }
-}
+impl_header!(Date, "Date", HttpDate);
 
 bench_header!(imf_fixdate, Date, { vec![b"Sun, 07 Nov 1994 08:48:37 GMT".to_vec()] });
 bench_header!(rfc_850, Date, { vec![b"Sunday, 06-Nov-94 08:49:37 GMT".to_vec()] });