Fix Bytes::is_unique when created from shared BytesMut (#718)

The `is_unique` entry in the vtable for `Bytes` created from a shared
`BytesMut` just called the `shared_is_unique` function from the `bytes`
module. However, that function dereferences the `data` argument` as
`bytes::Shared`, but the actual underlying type is `bytes_mut::Shared`.

tests/test_bytes.rs

@@ -1172,3 +1172,12 @@ fn shared_is_unique() {
     drop(b);
     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());
+}