__contains__ does not work with DataArray

#### Code Sample, a copy-pastable example if possible

```python
>>> da = xr.DataArray([0, 1, 2], dims='x')
>>> 0 in da
( warning omitted )
False
>>> 0 in da.values
True
```

#### Problem description

`__contains__` should work as np.ndarray does.

#### Expected Output

```python
>>> 0 in da
True
```