Fix saving a dataset with a prerequisites attrs to netcdf

[mraspaud]

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

[coveralls]

Coverage decreased (-0.004%) to 72.962% when pulling 0750e5c on fix-nc-writer-prereqs into 2f45b54 on master.

[codecov]

Codecov Report

Merging #448 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #448      +/-   ##
==========================================
+ Coverage   72.96%   73.11%   +0.14%     
==========================================
  Files         134      134              
  Lines       17689    17789     +100     
==========================================
+ Hits        12907    13006      +99     
- Misses       4782     4783       +1

Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_cf.py	97.89% <100%> (+0.06%)	⬆️
satpy/writers/cf_writer.py	68.18% <100%> (+0.73%)	⬆️
satpy/tests/test_readers.py	98.82% <0%> (+0.3%)	⬆️
satpy/tests/reader_tests/test_hdf5_utils.py	93.47% <0%> (+0.94%)	⬆️

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[djhoese]

@mraspaud Ready for merge? Feel free to merge it.