Gix-date: gix-date: undefined behavior due to invalid string generation

@@ -43,22 +43,12 @@ impl TimeBuf {
     }
 }
 
-impl std::io::Write for TimeBuf {
-    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
-        self.buf.write(buf)
-    }
-
-    fn flush(&mut self) -> std::io::Result<()> {
-        self.buf.flush()
-    }
-}
-
 impl Time {
     /// Serialize this instance into `buf`, exactly as it would appear in the header of a Git commit,
     /// and return `buf` as `&str` for easy consumption.
     pub fn to_str<'a>(&self, buf: &'a mut TimeBuf) -> &'a str {
         buf.clear();
-        self.write_to(buf)
+        self.write_to(&mut buf.buf)
             .expect("write to memory of just the right size cannot fail");
         buf.as_str()
     }

@@ -109,13 +109,15 @@ mod write_to {
 
     #[test]
     fn max() -> gix_testtools::Result {
-        let mut buf = TimeBuf::default();
+        let mut buf = Vec::new();
         Time::MAX.write_to(&mut buf)?;
         assert_eq!(Time::MAX.size(), 25, "The largest possible serialized size");
 
         let expected = "9223372036854775807 +9959";
-        assert_eq!(buf.as_str(), expected);
-        assert_eq!(buf.as_str().len(), Time::MAX.size());
+        assert_eq!(buf.as_bstr(), expected);
+        assert_eq!(buf.len(), Time::MAX.size());
+
+        let mut buf = TimeBuf::default();
         assert_eq!(Time::MAX.to_str(&mut buf), expected);
         Ok(())
     }