Use Formatter::pad (instead of write_str) for Weekdays

That way, width, alignment and fill from the format string are
respected.

Fixes #1620.

src/weekday.rs

@@ -171,7 +171,7 @@ impl Weekday {
 
 impl fmt::Display for Weekday {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        f.write_str(match *self {
+        f.pad(match *self {
             Weekday::Mon => "Mon",
             Weekday::Tue => "Tue",
             Weekday::Wed => "Wed",
@@ -331,6 +331,16 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_formatting_alignment() {
+        // No exhaustive testing here as we just delegate the
+        // implementation to Formatter::pad. Just some basic smoke
+        // testing to ensure that it's in fact being done.
+        assert_eq!(format!("{:x>7}", Weekday::Mon), "xxxxMon");
+        assert_eq!(format!("{:^7}", Weekday::Mon), "  Mon  ");
+        assert_eq!(format!("{:Z<7}", Weekday::Mon), "MonZZZZ");
+    }
+
     #[test]
     #[cfg(feature = "serde")]
     fn test_serde_serialize() {