[master] Fix automatic type conversion on nested fields

[codecov-io]

Codecov Report

Merging #834 into master will decrease coverage by 0.1%. The diff coverage is 100%.

@@           Coverage Diff            @@
##           master    #834     +/-   ##
========================================
- Coverage    94.3%   94.3%   -<.1%     
========================================
  Files         312     312             
  Lines       28300   28318     +18     
========================================
+ Hits        26686   26702     +16     
- Misses       1614    1616      +2

Impacted Files	Coverage Δ
guillotina/tests/test_api.py	99.6% <100%> (ø)	:arrow_up:
guillotina/json/deserialize_value.py	90% <100%> (-0.8%)	:arrow_down:
guillotina/tests/test_serialize.py	100% <100%> (ø)	:arrow_up:

[vangheem]

@masipcat so I don't know about this anymore. Did you see the problem I noticed? (this is a problem from even before I made the optimizations)