Fix 'observed' kwarg not doing anything on SeriesGroupBy

[krsnik93]

• closes observed keyword for SeriesGroupBy Ignored #24880
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[pep8speaks]

Hello @krsnik93! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2019-05-19 21:28:27 UTC

[WillAyd]

I think there might be a simpler way of going about this. Does adding _reindex_output to _wrap_output in SeriesGroupBy fix it instead? I think the problem might just be that the DataFrameGroupBy wrap methods have that already but the SeriesGroupBy ones don't, so that could help

[codecov]

Codecov Report

Merging #26461 into master will decrease coverage by 50.05%.
The diff coverage is 12.9%.

@@             Coverage Diff             @@
##           master   #26461       +/-   ##
===========================================
- Coverage   91.74%   41.69%   -50.06%     
===========================================
  Files         174      174               
  Lines       50748    50751        +3     
===========================================
- Hits        46560    21161    -25399     
- Misses       4188    29590    +25402

Flag	Coverage Δ
#multiple	?
#single	41.69% <12.9%> (-0.09%)	⬇️

Impacted Files	Coverage Δ
pandas/core/groupby/generic.py	13% <12.5%> (-75.96%)	⬇️
pandas/core/groupby/groupby.py	23.94% <13.04%> (-73.3%)	⬇️
pandas/io/formats/latex.py	0% <0%> (-100%)	⬇️
pandas/io/sas/sas_constants.py	0% <0%> (-100%)	⬇️
pandas/core/groupby/categorical.py	0% <0%> (-100%)	⬇️
pandas/tseries/plotting.py	0% <0%> (-100%)	⬇️
pandas/tseries/converter.py	0% <0%> (-100%)	⬇️
pandas/io/formats/html.py	0% <0%> (-99.37%)	⬇️
pandas/io/sas/sas7bdat.py	0% <0%> (-91.16%)	⬇️
pandas/io/sas/sas_xport.py	0% <0%> (-90.1%)	⬇️
... and 131 more

---

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[codecov]

Codecov Report

Merging #26461 into master will decrease coverage by <.01%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##           master   #26461      +/-   ##
==========================================
- Coverage   91.74%   91.74%   -0.01%     
==========================================
  Files         174      174              
  Lines       50748    50751       +3     
==========================================
  Hits        46560    46560              
- Misses       4188     4191       +3

Flag	Coverage Δ
#multiple	90.25% <96.77%> (ø)	⬆️
#single	41.71% <12.9%> (-0.07%)	⬇️

Impacted Files	Coverage Δ
pandas/core/groupby/generic.py	88.81% <100%> (-0.16%)	⬇️
pandas/core/groupby/groupby.py	97.18% <95.65%> (-0.06%)	⬇️
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	97.02% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.7% <0%> (+0.1%)	⬆️

---

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[jreback]

@krsnik93 this PR might be ok