Split common headers into a submodule and into their own files

This is a more extensible place to put them and doesn't clutter up
header/mod.rs as much as the old scheme did.

Fixes #8

src/header/common/date.rs

@@ -0,0 +1,66 @@
+use header::Header;
+use std::fmt::{mod, Show};
+use super::from_one_raw_str;
+use std::from_str::FromStr;
+use time::{Tm, strptime};
+
+// Egh, replace as soon as something better than time::Tm exists.
+/// The `Date` header field.
+#[deriving(PartialEq, Clone)]
+pub struct Date(pub Tm);
+
+impl Header for Date {
+    fn header_name(_: Option<Date>) -> &'static str {
+        "date"
+    }
+
+    fn parse_header(raw: &[Vec<u8>]) -> Option<Date> {
+        from_one_raw_str(raw)
+    }
+
+    fn fmt_header(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        self.fmt(fmt)
+    }
+}
+
+impl fmt::Show for Date {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        let Date(ref tm) = *self;
+        // bummer that tm.strftime allocates a string. It would nice if it
+        // returned a Show instead, since I don't need the String here
+        write!(fmt, "{}", tm.to_utc().rfc822())
+    }
+}
+
+impl FromStr for Date {
+    //    Prior to 1995, there were three different formats commonly used by
+    //   servers to communicate timestamps.  For compatibility with old
+    //   implementations, all three are defined here.  The preferred format is
+    //   a fixed-length and single-zone subset of the date and time
+    //   specification used by the Internet Message Format [RFC5322].
+    //
+    //     HTTP-date    = IMF-fixdate / obs-date
+    //
+    //   An example of the preferred format is
+    //
+    //     Sun, 06 Nov 1994 08:49:37 GMT    ; IMF-fixdate
+    //
+    //   Examples of the two obsolete formats are
+    //
+    //     Sunday, 06-Nov-94 08:49:37 GMT   ; obsolete RFC 850 format
+    //     Sun Nov  6 08:49:37 1994         ; ANSI C's asctime() format
+    //
+    //   A recipient that parses a timestamp value in an HTTP header field
+    //   MUST accept all three HTTP-date formats.  When a sender generates a
+    //   header field that contains one or more timestamps defined as
+    //   HTTP-date, the sender MUST generate those timestamps in the
+    //   IMF-fixdate format.
+    fn from_str(s: &str) -> Option<Date> {
+        strptime(s, "%a, %d %b %Y %T %Z").or_else(|_| {
+            strptime(s, "%A, %d-%b-%y %T %Z")
+        }).or_else(|_| {
+            strptime(s, "%c")
+        }).ok().map(|tm| Date(tm))
+    }
+}
+