Fix NumPy matrix bug.

[hameerabbasi]

Fixes #199

[codecov]

Codecov Report

Merging #200 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #200   +/-   ##
=======================================
  Coverage   97.65%   97.65%           
=======================================
  Files          11       11           
  Lines        1447     1447           
=======================================
  Hits         1413     1413           
  Misses         34       34

Impacted Files	Coverage Δ
sparse/coo/core.py	97.14% <100%> (ø)	⬆️

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