Array.read_only incorrect

```python
import zarr
import zarr.storage

store = zarr.storage.MemoryStore()
zarr.create((100,), store=store, zarr_format=2, path="a")


arr = zarr.open_array(store=store, path="a", zarr_format=2, mode="r")
print(arr.read_only)
# False
```

Given the mode is "r", I think `.read_only` should return `True`?