Fix masked resampling when dataset dtype is integer

[mraspaud]

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[djhoese]

Makes sense I think.

[codecov]

Codecov Report

Merging #662 into master will increase coverage by 0.02%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master     #662      +/-   ##
==========================================
+ Coverage   78.61%   78.63%   +0.02%     
==========================================
  Files         138      138              
  Lines       20325    20332       +7     
==========================================
+ Hits        15979    15989      +10     
+ Misses       4346     4343       -3

Impacted Files	Coverage Δ
satpy/tests/test_resample.py	98.91% <100%> (+0.01%)	⬆️
satpy/resample.py	88.02% <75%> (-0.18%)	⬇️
satpy/readers/__init__.py	94.54% <0%> (+1.21%)	⬆️

---

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 be6e5a0...c383dbd. Read the comment docs.

[coveralls]

Coverage decreased (-0.002%) to 78.616% when pulling c383dbd on mraspaud:fix-resample-int-mask into be6e5a0 on pytroll:master.

[coveralls]

Coverage increased (+0.02%) to 78.635% when pulling c383dbd on mraspaud:fix-resample-int-mask into be6e5a0 on pytroll:master.