MapBuilder retry_failed bug

[acrutt]

Bug fix so that retry_failed=True argument of MapBuilder does result in attempting to process failed docs in the target store.

[codecov]

Codecov Report

Merging #111 into master will decrease coverage by 0.7%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
- Coverage   81.57%   80.87%   -0.71%     
==========================================
  Files          21       21              
  Lines        1563     1563              
==========================================
- Hits         1275     1264      -11     
- Misses        288      299      +11

Impacted Files	Coverage Δ
src/maggma/builders/map_builder.py	91.2% <ø> (ø)	⬆️
src/maggma/stores/mongolike.py	88.48% <0%> (-5.76%)	⬇️

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[shyamd]

Can you add a test for this so we don't miss it in the future?

[acrutt]

I can work on a test but will need more instructions/guidance of what needs to be done since I haven't made one before.

[shyamd]

You might actually write tests for your ProjectionBuilder. You can model it off the copybuilder or group builder tests. The idea is to make a mock data set, use memory stores to run the builder and make sure all the different functionality works. Feel free to write up something and push the changes here. I can comment on what to do after that.

https://github.com/materialsproject/maggma/blob/master/tests/builders/test_group_builder.py