ApplyTypeAnnotationsVisitor: fix default value

[hauntsaninja]

Summary

ApplyTypeAnnotationsVisitor erases default values of positional-only and keyword-only args. See Instagram/MonkeyType#198 for a repro

Test Plan

Added tests for the relevant issues in test_apply_type_annotations.py

[codecov-commenter]

Codecov Report

Merging #314 into master will decrease coverage by 0.01%.
The diff coverage is 62.50%.

@@            Coverage Diff             @@
##           master     #314      +/-   ##
==========================================
- Coverage   93.96%   93.95%   -0.02%     
==========================================
  Files         223      223              
  Lines       21438    21446       +8     
==========================================
+ Hits        20145    20150       +5     
- Misses       1293     1296       +3     

Impacted Files	Coverage Δ
libcst/codemod/visitors/_apply_type_annotations.py	95.83% <ø> (ø)
...emod/visitors/tests/test_apply_type_annotations.py	89.65% <62.50%> (-10.35%)	⬇️

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[jimmylai]

LGTM
@hauntsaninja Nice catch and thanks for the fix!

[hauntsaninja]

Thanks for the quick review! I was wondering what LibCST's release process is like? This feels like a fairly major regression for MonkeyType (which apparently only recently started using LibCST), so might be nice to cut a new release soon

[jimmylai]

Thanks for the quick review! I was wondering what LibCST's release process is like? This feels like a fairly major regression for MonkeyType (which apparently only recently started using LibCST), so might be nice to cut a new release soon

We currently release occasionally and we do prioritize critical bug fixes. There are currently other fixes on master and @josieesh can make a release later this week.