Code Sample, a copy-pastable example if possible

```python da = xr.DataArray(np.array([1,2,3,4], dtype=np.float).reshape(2,2), ... coords=[pd.date_range('1/1/2000', '1/2/2000', freq='D'), ... np.linspace(0,1,num=2)], ... dims=['time', 'latitude'])

da.resample(freq='M', dim='time', how='mean')

/Users/Ray/anaconda/envs/rot-eof-dev-env/bin/ipython:1: DeprecationWarning:

.resample() has been modified to defer calculations. Instead of passing 'dim' and 'how="mean", #instead consider using .resample(time="M").mean()

#!/Users/Ray/anaconda/envs/rot-eof-dev-env/bin/python

Out[66]:

<xarray.DataArray (time: 1, latitude: 2)>

array([[2., 3.]])

Coordinates:

* time (time) datetime64[ns] 2000-01-31

* latitude (latitude) float64 0.0 1.0

da.resample(time="M").mean()

<xarray.DataArray (time: 1)>

array([2.5])

Coordinates:

* time (time) datetime64[ns] 2000-01-31

```

Problem description

The DeprecationWarning example seems to only work for 1d arrays as it doesn't average along any dimension.

A quick fix could be to show the warning only if the DataArray/Dataset is 1D.

A more thorough fix could be to wrap .resample(time="M").mean() as .resample(freq='M', dim='time', how='mean')???

Expected Output

Same as da.resample(freq='M', dim='time', how='mean')

Output of xr.show_versions()

xr.show_versions() # Not sure about the h5py FutureWarning? /Users/Ray/anaconda/envs/rot-eof-dev-env/lib/python3.6/site-packages/h5py/__init__.py:36: FutureWarning: Conversion of the second argument of issubdtype from `float` to `np.floating` is deprecated. In future, it will be treated as `np.float64 == np.dtype(float).type`. from ._conv import register_converters as _register_converters INSTALLED VERSIONS ------------------ commit: None python: 3.6.5.final.0 python-bits: 64 OS: Darwin OS-release: 17.5.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 xarray: 0.10.3 pandas: 0.22.0 numpy: 1.14.2 scipy: 1.0.1 netCDF4: 1.3.1 h5netcdf: 0.5.1 h5py: 2.7.1 Nio: None zarr: None bottleneck: 1.2.1 cyordereddict: None dask: 0.17.2 distributed: 1.21.6 matplotlib: 2.2.2 cartopy: 0.16.0 seaborn: None setuptools: 39.0.1 pip: 9.0.3 conda: None pytest: None IPython: 6.3.1 sphinx: None