Add DATE header constant

Author: technetos

src/headers/constants.rs

@@ -14,3 +14,6 @@ pub const SET_COOKIE: HeaderName = HeaderName::from_lowercase_str("set-cookie");
 
 /// The `Transfer-Encoding` Header
 pub const TRANSFER_ENCODING: HeaderName = HeaderName::from_lowercase_str("transfer-encoding");
+
+/// The `Date` Header
+pub const DATE: HeaderName = HeaderName::from_lowercase_str("date");