Fix SCMI writer not writing fill values properly

[djhoese]

@rayg-ssec found out that SCMI files generated did not respect fill values from the original data. I'm not exactly sure how this ever worked.

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

[coveralls]

Coverage decreased (-0.02%) to 72.953% when pulling 232fd53 on djhoese:bugfix-scmi-masked into 38fb064 on pytroll:master.

[codecov]

Codecov Report

Merging #456 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #456   +/-   ##
=======================================
  Coverage   72.97%   72.97%           
=======================================
  Files         134      134           
  Lines       17695    17695           
=======================================
  Hits        12913    12913           
  Misses       4782     4782

Impacted Files	Coverage Δ
satpy/writers/scmi.py	72.18% <100%> (ø)	⬆️
satpy/readers/clavrx.py	91.77% <100%> (ø)	⬆️

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