[easy] - fixing all expression matrix saves

[shanaxel42]

Should fix #1411, also added small test

[codecov-io]

Codecov Report

Merging #1413 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1413      +/-   ##
==========================================
+ Coverage   89.42%   89.44%   +0.01%     
==========================================
  Files         152      152              
  Lines        5531     5531              
==========================================
+ Hits         4946     4947       +1     
+ Misses        585      584       -1

Impacted Files	Coverage Δ
starfish/core/intensity_table/intensity_table.py	97.14% <ø> (+0.71%)	⬆️
starfish/core/types/_constants.py	98.11% <0%> (-1.89%)	⬇️
...arfish/core/expression_matrix/expression_matrix.py	50% <0%> (+4.16%)	⬆️

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[ttung]

note that travis is not green, because the test environment does not have anndata installed.