conversion of described classes supports private members

[grisumbras]

Fix #986

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/988/pullrequest.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/988/pullrequest.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.08%. Comparing base (8f5b635) to head (2883ff1).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #988   +/-   ##
========================================
  Coverage    93.08%   93.08%           
========================================
  Files           87       87           
  Lines         8125     8125           
========================================
  Hits          7563     7563           
  Misses         562      562           

Files	Coverage Δ
include/boost/json/detail/parse_into.hpp	100.00% <ø> (ø)
include/boost/json/detail/value_from.hpp	100.00% <ø> (ø)
include/boost/json/detail/value_to.hpp	100.00% <ø> (ø)
include/boost/json/impl/conversion.hpp	100.00% <ø> (ø)

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/988/pullrequest.html