Fix nanmean for complex arrays.

[hameerabbasi]

Fixes #553

[codecov[bot]]

Codecov Report

Merging #555 (bf24a24) into master (30b7172) will decrease coverage by 0.44%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
- Coverage   92.75%   92.30%   -0.45%     
==========================================
  Files          20       20              
  Lines        3120     3120              
==========================================
- Hits         2894     2880      -14     
- Misses        226      240      +14     

[keewis]

it seems complex128 has the same issue, so it might be good to check that as well?

[hameerabbasi]

it seems complex128 has the same issue, so it might be good to check that as well?

Thanks, done in https://github.com/pydata/sparse/pull/555/commits/4cdf8c0cbeb81eeb48bb01fe4963312875c52ba6.