Merge 'v1.6.1' into 'master' (#721)

tests/test_bytes.rs

@@ -1173,6 +1173,15 @@ fn shared_is_unique() {
     assert!(c.is_unique());
 }
 
+#[test]
+fn mut_shared_is_unique() {
+    let mut b = BytesMut::from(LONG);
+    let c = b.split().freeze();
+    assert!(!c.is_unique());
+    drop(b);
+    assert!(c.is_unique());
+}
+
 #[test]
 fn test_bytesmut_from_bytes_static() {
     let bs = b"1b23exfcz3r";