Fix the scalapb json marshaller for sealed-oneof messages

[ateirney]

When returning a ScalaPB message of the sealed-oneof flavour,
the JSON marshaller does not marshal the wrapping message.

This changeset corrects that by detecting the case where the value passed in
is a GeneratedSealedOneof, and then marshalls the container message.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #3342 (6fded71) into master (d2d6207) will increase coverage by 0.02%.
The diff coverage is 84.61%.

@@             Coverage Diff              @@
##             master    #3342      +/-   ##
============================================
+ Coverage     74.16%   74.18%   +0.02%     
- Complexity    13276    13290      +14     
============================================
  Files          1158     1159       +1     
  Lines         50626    50740     +114     
  Branches       6491     6504      +13     
============================================
+ Hits          37548    37643      +95     
- Misses         9765     9778      +13     
- Partials       3313     3319       +6     

Impacted Files	Coverage Δ	Complexity Δ
...rp/armeria/client/WebClientRequestPreparation.java	30.00% <0.00%> (-1.04%)	7.00 <0.00> (ø)
...in/example/armeria/grpc/kotlin/HelloServiceImpl.kt	75.86% <ø> (+3.31%)	7.00 <0.00> (-3.00)	⬆️
...meria/spring/AbstractArmeriaAutoConfiguration.java	64.70% <0.00%> (ø)	6.00 <0.00> (ø)
.../spring/ArmeriaBeanPostProcessorConfiguration.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...meria/internal/common/grpc/HttpStreamDeframer.java	67.34% <50.00%> (-1.55%)	11.00 <1.00> (ø)
...inecorp/armeria/server/grpc/FramedGrpcService.java	82.17% <50.00%> (-0.51%)	30.00 <0.00> (ø)
.../linecorp/armeria/common/stream/StreamMessage.java	80.76% <57.14%> (-3.68%)	22.00 <2.00> (+2.00)	⬇️
...armeria/internal/common/thrift/ThriftFunction.java	86.15% <78.57%> (-1.25%)	51.00 <3.00> (+2.00)	⬇️
...p/armeria/common/stream/FuseableStreamMessage.java	87.64% <87.64%> (ø)	10.00 <10.00> (?)
...orp/armeria/common/AbstractHttpRequestBuilder.java	87.21% <90.90%> (+0.33%)	50.00 <1.00> (+3.00)
... and 33 more

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[trustin]

@andrew-teirney Gentle ping

[trustin]

Thanks, @andrew-teirney and @ikhoon !

[ikhoon]

Thanks! @andrew-teirney

[minwoox]

Thanks, @andrew-teirney and @ikhoon !!!