Fix grid argument handling in overlaying

[mraspaud]

This was introduced in #934

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage increased (+0.02%) to 86.771% when pulling 72edec6 on mraspaud:fix-grid-font into e7c56ca on pytroll:master.

[codecov]

Codecov Report

Merging #959 into master will increase coverage by 0.01%.
The diff coverage is 86.36%.

@@            Coverage Diff             @@
##           master     #959      +/-   ##
==========================================
+ Coverage   86.75%   86.77%   +0.01%     
==========================================
  Files         179      179              
  Lines       27279    27296      +17     
==========================================
+ Hits        23665    23685      +20     
+ Misses       3614     3611       -3

Impacted Files	Coverage Δ
satpy/tests/test_writers.py	98.01% <100%> (+0.09%)	⬆️
satpy/writers/__init__.py	85.67% <50%> (+0.77%)	⬆️

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