set normalize_keys=False in fsstore constructor

[d-v-b]

Set normalize_keys to False by default in FSStore. Addresses #841

[codecov]

Codecov Report

Merging #842 (46094a0) into master (78eb8b7) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #842   +/-   ##
=======================================
  Coverage   99.94%   99.94%           
=======================================
  Files          31       31           
  Lines       10936    10936           
=======================================
  Hits        10930    10930           
  Misses          6        6           

Impacted Files	Coverage Δ
zarr/storage.py	100.00% <100.00%> (ø)

[d-v-b]

I think if we are going to require that constructors take the same default argument, there should be a test for it (such a test would have prevented me from making this change in the first place). This would require changing all the store test classes, so I deemed it out of scope for this patch.

[joshmoore]

Thanks for the quick PR, @d-v-b. Sorry for the slow release (I was traveling! 😉) Going to turn this into 2.10.1 now.

Regarding the constructor arguments, will it help if we define the defaults somewhere as constants?