Update lib.rs (#75)

For ergonomic reasons a default implementation for `DateTime` is useful and all zero's and false is fine, though some specific other value may be more suitable

Author: gdennie

src/lib.rs

@@ -505,7 +505,7 @@ pub struct ContentType<'x> {
 }
 
 /// An RFC5322 datetime.
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Default)]
 #[cfg_attr(feature = "serde_support", derive(Serialize, Deserialize))]
 pub struct DateTime {
     pub year: u16,