Add some tests for Limit, BytesMut and Reader (#785)

tests/test_bytes.rs

@@ -186,6 +186,13 @@ fn split_off_oob() {
     let _ = hello.split_off(44);
 }
 
+#[test]
+#[should_panic = "split_off out of bounds"]
+fn bytes_mut_split_off_oob() {
+    let mut hello = BytesMut::from(&b"helloworld"[..]);
+    let _ = hello.split_off(44);
+}
+
 #[test]
 fn split_off_uninitialized() {
     let mut bytes = BytesMut::with_capacity(1024);