feat(headers): Add If-Range header

Closes #388

src/header/common/if_range.rs

@@ -0,0 +1,73 @@
+use header::{self, EntityTag, HttpDate};
+
+/// `If-Range` header, defined in [RFC7233](http://tools.ietf.org/html/rfc7233#section-3.2)
+///
+/// If a client has a partial copy of a representation and wishes to have
+/// an up-to-date copy of the entire representation, it could use the
+/// Range header field with a conditional GET (using either or both of
+/// If-Unmodified-Since and If-Match.)  However, if the precondition
+/// fails because the representation has been modified, the client would
+/// then have to make a second request to obtain the entire current
+/// representation.
+///
+/// The `If-Range` header field allows a client to \"short-circuit\" the
+/// second request.  Informally, its meaning is as follows: if the
+/// representation is unchanged, send me the part(s) that I am requesting
+/// in Range; otherwise, send me the entire representation.
+///
+/// # ABNF
+/// ```plain
+/// If-Range = entity-tag / HTTP-date
+/// ```
+///
+/// # Example values
+/// * `Sat, 29 Oct 1994 19:43:31 GMT`
+/// * `\"xyzzy\"`
+#[derive(Clone, Debug, PartialEq)]
+pub enum IfRange {
+    /// The entity-tag the client has of the resource
+    EntityTag(EntityTag),
+    /// The date when the client retrieved the resource
+    Date(HttpDate),
+}
+
+impl header::Header for IfRange {
+    fn header_name() -> &'static str {
+        "If-Range"
+    }
+    fn parse_header(raw: &[Vec<u8>]) -> Option<IfRange> {
+        let etag: Option<EntityTag> = header::parsing::from_one_raw_str(raw);
+        if etag != None {
+            return Some(IfRange::EntityTag(etag.unwrap()));
+        }
+        let date: Option<HttpDate> = header::parsing::from_one_raw_str(raw);
+        if date != None {
+            return Some(IfRange::Date(date.unwrap()));
+        }
+        None
+    }
+}
+
+impl header::HeaderFormat for IfRange {
+    fn fmt_header(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+        match self {
+            &IfRange::EntityTag(ref x) => write!(f, "{}", x),
+            &IfRange::Date(ref x) => write!(f, "{}", x),
+        }
+    }
+}
+
+impl ::std::fmt::Display for IfRange {
+    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+        use header::HeaderFormat;
+        self.fmt_header(f)
+    }
+}
+
+#[cfg(test)]
+mod test_range {
+    use header::*;
+    use super::IfRange as HeaderField;
+    test_header!(test1, vec![b"Sat, 29 Oct 1994 19:43:31 GMT"]);
+    test_header!(test2, vec![b"\"xyzzy\""]);
+}