Fixed str::raw::push_byte

It was previously pushing the byte on top of the string's null
terminator. I added a test to make sure it doesn't break in the future.

src/libstd/str.rs

@@ -878,7 +878,7 @@ pub mod raw {
         let new_len = s.len() + 1;
         s.reserve_at_least(new_len);
         do s.as_mut_buf |buf, len| {
-            *ptr::mut_offset(buf, len as int) = b;
+            *ptr::mut_offset(buf, (len-1) as int) = b;
         }
         set_len(&mut *s, new_len);
     }
@@ -2801,6 +2801,13 @@ mod tests {
         assert!(!"   _   ".is_whitespace());
     }
 
+    #[test]
+    fn test_push_byte() {
+        let mut s = ~"ABC";
+        unsafe{raw::push_byte(&mut s, 'D' as u8)};
+        assert_eq!(s, ~"ABCD");
+    }
+
     #[test]
     fn test_shift_byte() {
         let mut s = ~"ABC";