Disallow auto-densification.

[hameerabbasi]

Fixes #218

[codecov]

Codecov Report

Merging #220 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   97.59%   97.59%   +<.01%     
==========================================
  Files          11       11              
  Lines        1539     1541       +2     
==========================================
+ Hits         1502     1504       +2     
  Misses         37       37

Impacted Files	Coverage Δ
sparse/coo/core.py	96.66% <ø> (-0.05%)	⬇️
sparse/sparse_array.py	97.36% <100%> (+0.59%)	⬆️
sparse/coo/common.py	98.11% <100%> (ø)	⬆️

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[scopatz]

LGTM! Thanks @hameerabbasi!