Preserve values from being formatted

[sliverc]

Fixes #313

Description of the Change

According to Json API Spec should only field names (hence the name JSON_API_FORMAT_FIELD_NAMES) be specifically named resp. formatted. The value itself may contain any json value though and we should keep it as is.

With the option JSON_API_FORMAT_KEYS all keys (incl. value keys) are formatted though. This leads to underscored keys of a attribute value in Django code (or even on the database when using JSONField)

To fix this I have introduced JSON_API_FIELD_NAMES setting which preserves value from being formatted. JSON_API_FORMAT_KEYS still works as before though for backwards compatibility but is marked deprecated.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added (not needed as only build change)
• documentation updated (not needed as only build change)
• changelog entry added to CHANGELOG.md
• author name in AUTHORS

[codecov-io]

Codecov Report

Merging #420 into master will increase coverage by 0.08%.
The diff coverage is 98.52%.

@@            Coverage Diff             @@
##           master     #420      +/-   ##
==========================================
+ Coverage    92.6%   92.68%   +0.08%     
==========================================
  Files          54       54              
  Lines        2812     2858      +46     
==========================================
+ Hits         2604     2649      +45     
- Misses        208      209       +1

Impacted Files	Coverage Δ
example/settings/dev.py	100% <100%> (ø)	⬆️
example/settings/test.py	100% <100%> (ø)	⬆️
rest_framework_json_api/parsers.py	97.14% <100%> (ø)	⬆️
rest_framework_json_api/renderers.py	85.32% <100%> (ø)	⬆️
example/tests/unit/test_renderers.py	100% <100%> (ø)	⬆️
example/tests/test_parsers.py	100% <100%> (ø)	⬆️
example/tests/test_generic_viewset.py	100% <100%> (ø)	⬆️
example/tests/test_model_viewsets.py	100% <100%> (ø)	⬆️
example/tests/unit/test_settings.py	100% <100%> (ø)	⬆️
example/api/resources/identity.py	94.28% <100%> (ø)	⬆️
... and 3 more

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[n2ygk]

Did the existing format_keys() also format the values? I was not aware, but I see where it recurses down the tree.

Good that you've fixed it.