BUG: propagate dropna in pd.Grouper
- [x] closes #36620
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

A precursor to #35751
I noticed that the `Grouper` docstring didn't have a `dropna` arg so I added that here.

I said `added in 1.2.0` - a bit of a slight of hand since it the attribute was already there in the code in `Grouper` (but it didn't exist/propagate in `BaseGrouper` so it was unusable) The test is fine it was a general question in the PR. Was addressed by Jeff as well so otherwise lgtm

Sent from my iPhone

> On Sep 25, 2020, at 11:49 AM, Richard Shadrach <notifications@github.com> wrote:
> 
> ﻿
> @rhshadrach commented on this pull request.
> 
> In pandas/tests/groupby/test_groupby_dropna.py:
> 
> > @@ -162,6 +162,13 @@ def test_groupby_dropna_series_by(dropna, expected):
>      tm.assert_series_equal(result, expected)
>  
>  
> +@pytest.mark.parametrize("dropna", (False, True))
> +def test_grouper_dropna_propagation(dropna):
> +    df = pd.DataFrame({"A": [0, 0, 1, None], "B": [1, 2, 3, None]})
> +    gb = df.groupby("A", dropna=dropna)
> +    assert gb.grouper.dropna == dropna
> @WillAyd maybe you're thinking we should only be testing the public API and not any internals?
> 
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 ok this is fine