enable conversion for derived described structs

[grisumbras]

Fix #1086.

[cppalliance-bot]

An automated preview of the documentation is available at https://1088.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1088/pullrequest.html

[cppalliance-bot]

An automated preview of the documentation is available at https://1088.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1088/pullrequest.html

[cppalliance-bot]

An automated preview of the documentation is available at https://1088.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1088/pullrequest.html

[cppalliance-bot]

An automated preview of the documentation is available at https://1088.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

[cppalliance-bot]

An automated preview of the documentation is available at https://1088.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1088/pullrequest.html

[cppalliance-bot]

An automated preview of the documentation is available at https://1088.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.70%. Comparing base (6c79a2c) to head (bfedf3c). Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1088   +/-   ##
========================================
  Coverage    93.70%   93.70%           
========================================
  Files           91       91           
  Lines         9136     9136           
========================================
  Hits          8561     8561           
  Misses         575      575           

Files with missing lines	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)
include/boost/json/impl/serializer.hpp	96.40% <ø> (ø)

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1088/pullrequest.html