Allow (clippy) needless_pass_by_value on signed_duration_since

Current clippy is probably correct, but its a breaking change that
isn't appropriate now. Add allow's to get the build working again.

Also these Date(Time)::signed_duration_since cases appear to match
there `Naive` counterparts, where clippy isn't complaining. If its
fixed in the future, should probably be changed across the board, not
just here.

src/date.rs

@@ -224,6 +224,7 @@ impl<Tz: TimeZone> Date<Tz> {
     ///
     /// This does not overflow or underflow at all,
     /// as all possible output fits in the range of `Duration`.
+    #[cfg_attr(feature = "cargo-clippy", allow(needless_pass_by_value))]
     #[inline]
     pub fn signed_duration_since<Tz2: TimeZone>(self, rhs: Date<Tz2>) -> OldDuration {
         self.date.signed_duration_since(rhs.date)

src/datetime.rs

@@ -159,6 +159,7 @@ impl<Tz: TimeZone> DateTime<Tz> {
 
     /// Subtracts another `DateTime` from the current date and time.
     /// This does not overflow or underflow at all.
+    #[cfg_attr(feature = "cargo-clippy", allow(needless_pass_by_value))]
     #[inline]
     pub fn signed_duration_since<Tz2: TimeZone>(self, rhs: DateTime<Tz2>) -> OldDuration {
         self.datetime.signed_duration_since(rhs.datetime)