Fix dtype not being preserved with the out kwarg.

[hameerabbasi]

Fix #205

[codecov]

Codecov Report

Merging #206 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
+ Coverage   97.63%   97.66%   +0.02%     
==========================================
  Files          11       11              
  Lines        1483     1501      +18     
==========================================
+ Hits         1448     1466      +18     
  Misses         35       35

Impacted Files	Coverage Δ
sparse/coo/umath.py	97.13% <100%> (+0.2%)	⬆️
sparse/coo/core.py	97.15% <100%> (+0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5333d7c...1402215. Read the comment docs.