set defaultLayer to "public.default" when its available

[typemytype]

fixing #674

[codecov[bot]]

Codecov Report

Merging #675 (3645fee) into master (3f6fdfe) will decrease coverage by 0.02%. The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master     #675      +/-   ##
==========================================
- Coverage   77.85%   77.83%   -0.03%     
==========================================
  Files          41       41              
  Lines        6100     6103       +3     
  Branches      979      980       +1     
==========================================
+ Hits         4749     4750       +1     
- Misses       1146     1148       +2     
  Partials      205      205              

Impacted Files	Coverage Δ
Lib/fontParts/base/font.py	56.59% <25.00%> (-0.15%)	:arrow_down:

[typesupply]

This logic makes sense to me.