Fix regression in Bytes::truncate (#333)

When the length to truncate is greater than the buffer's current
length, do nothing instead of clearing the contents.

tests/test_bytes.rs

@@ -312,6 +312,18 @@ fn split_off_to_at_gt_len() {
     }).is_err());
 }
 
+#[test]
+fn truncate() {
+    let s = &b"helloworld"[..];
+    let mut hello = Bytes::from(s);
+    hello.truncate(15);
+    assert_eq!(hello, s);
+    hello.truncate(10);
+    assert_eq!(hello, s);
+    hello.truncate(5);
+    assert_eq!(hello, "hello");
+}
+
 #[test]
 fn freeze_clone_shared() {
     let s = &b"abcdefgh"[..];