Fix heatmap norm=None issue

[stefanistrate]

Resolves #3275.

[mwaskom]

Thanks; seems reasonable aside from one nit, though a test would be nice too...

[codecov]

Codecov Report

Merging #3276 (8374046) into master (3733590) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3276   +/-   ##
=======================================
  Coverage   98.38%   98.38%           
=======================================
  Files          77       77           
  Lines       24696    24705    +9     
=======================================
+ Hits        24297    24306    +9     
  Misses        399      399           

Impacted Files	Coverage Δ
seaborn/matrix.py	96.18% <100.00%> (ø)
tests/test_matrix.py	97.69% <100.00%> (+0.02%)	⬆️

[stefanistrate]

Thanks; seems reasonable aside from one nit, though a test would be nice too...

Thanks for suggestion. I'll add a test soon.

[stefanistrate]

I've just added a test that fails before my change and passes after. Please take another look.

[mwaskom]

Test looks perfect, thanks!