Iterate over data_vars only

[max-sixty]

• Closes Iterating over a Dataset iterates only over its data_vars #884 (remove if there is no corresponding issue, which should only be the case for minor changes)
• Tests added (for all bug fixes or enhancements)
• Fully documented, including whats-new.rst for all changes and api.rst for new API (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[pep8speaks]

Hello @max-sixty! Thanks for submitting the PR.

• There are no PEP8 issues in the file xarray/core/dataset.py !

• There are no PEP8 issues in the file xarray/tests/test_dataset.py !

[max-sixty]

ref #2505

[max-sixty]

I'm seeing (unrelated) errors on AppVeyor only: https://ci.appveyor.com/project/shoyer/xray/builds/19763584/job/44ushvsfpuw77rqd

Collecting netcdf4 (from -r C:\projects\xray\ci\condaenv.z95tw7.requirements.txt (line 1))
  Downloading https://files.pythonhosted.org/packages/eb/aa/b067f3b1a2561f29f5c282d8a0f0f4bba5b13e9bdaa5fcd29005d226c448/netCDF4-1.4.2.tar.gz (769kB)
    Complete output from command python setup.py egg_info:
    reading from setup.cfg...
    
        HDF5_DIR environment variable not set, checking some standard locations ..
    checking C:\Users\appveyor ...
    checking /usr/local ...
    checking /sw ...
    checking /opt ...
    checking /opt/local ...
    checking /usr ...
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "c:\users\appveyor\appdata\local\temp\1\pip-install-nfiw_q\netcdf4\setup.py", line 371, in <module>
        _populate_hdf5_info(dirstosearch, inc_dirs, libs, lib_dirs)
      File "c:\users\appveyor\appdata\local\temp\1\pip-install-nfiw_q\netcdf4\setup.py", line 319, in _populate_hdf5_info
        raise ValueError('did not find HDF5 headers')
    ValueError: did not find HDF5 headers
    
    ----------------------------------------

[shoyer]

looks great -- thanks!