suppress the cast warning in java okhttp-gson client #869

Job	Run time
Build sbt/Scala client, servers (samples/client/petstore/java/okhttp-gson)	35s
Build sbt/Scala client, servers (samples/client/petstore/scalaz)	1m 59s
Build sbt/Scala client, servers (samples/client/petstore/scala-pekko)	34s
Build sbt/Scala client, servers (samples/client/petstore/scala-http4s)	42s
Build sbt/Scala client, servers (samples/server/petstore/scala-lagom-server)	5m 14s
Build sbt/Scala client, servers (samples/server/petstore/scala-play-server)	5m 4s
Build sbt/Scala client, servers (samples/server/petstore/scala-akka-http-server)	2m 0s
Build sbt/Scala client, servers (samples/server/petstore/scala-pekko-http-server)	40s
Build sbt/Scala client, servers (samples/server/petstore/scalatra)	3m 15s
Build sbt/Scala client, servers (samples/server/petstore/scala-finch)	3m 4s
Build sbt/Scala client, servers (samples/server/petstore/scala-http4s-server)	44s
Build sbt/Scala client, servers (samples/server/petstore/scala-cask)	1m 10s
	25m 1s

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