Fix DayNightCompositor masking imagery incorrectly

[pnuu]

This PR fixes the behaviour of DayNightCompositor, which was masking the imagery incorrectly.

• Closes Unmasked data when using DayNightCompositor #377
• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff

[coveralls]

Coverage remained the same at 69.085% when pulling 70218d9 on bugfix-dnc-masking into ade4f95 on master.

[codecov]

Codecov Report

Merging #378 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #378   +/-   ##
=======================================
  Coverage   69.08%   69.08%           
=======================================
  Files         123      123           
  Lines       15724    15724           
=======================================
  Hits        10863    10863           
  Misses       4861     4861

Impacted Files	Coverage Δ
satpy/composites/__init__.py	32.88% <0%> (ø)	⬆️

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