Format metadata field names correctly for OPTIONS request

[sliverc]

Supersedes #500

Description of the Change

Take into account setting JSON_API_FORMAT_FIELD_NAMES

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@b3cacb7). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #713   +/-   ##
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  Coverage          ?   97.08%           
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  Files             ?       54           
  Lines             ?     2742           
  Branches          ?        0           
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  Hits              ?     2662           
  Misses            ?       80           
  Partials          ?        0

Impacted Files	Coverage Δ
example/tests/test_format_keys.py	100% <100%> (ø)
rest_framework_json_api/metadata.py	91.66% <100%> (ø)

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[n2ygk]

LGTM. I suppose long-term we should discuss about whether OPTIONS should return an OAS schema or something more standard.