Suppress the cast warning in java okhttp-gson client #870

Job	Run time
Build sbt/Scala client, servers (samples/client/petstore/java/okhttp-gson)	37s
Build sbt/Scala client, servers (samples/client/petstore/scalaz)	2m 8s
Build sbt/Scala client, servers (samples/client/petstore/scala-pekko)	46s
Build sbt/Scala client, servers (samples/client/petstore/scala-http4s)	45s
Build sbt/Scala client, servers (samples/server/petstore/scala-lagom-server)	4m 9s
Build sbt/Scala client, servers (samples/server/petstore/scala-play-server)	4m 56s
Build sbt/Scala client, servers (samples/server/petstore/scala-akka-http-server)	2m 22s
Build sbt/Scala client, servers (samples/server/petstore/scala-pekko-http-server)	43s
Build sbt/Scala client, servers (samples/server/petstore/scalatra)	3m 6s
Build sbt/Scala client, servers (samples/server/petstore/scala-finch)	3m 7s
Build sbt/Scala client, servers (samples/server/petstore/scala-http4s-server)	46s
Build sbt/Scala client, servers (samples/server/petstore/scala-cask)	1m 6s
	24m 31s

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