Avoid UnsupportedEncodingException by design, replacing magic string "UTF-8" with StandardCharsets.UTF_8 #763

Job	Run time
Build sbt/Scala client, servers (samples/client/petstore/java/okhttp-gson)	36s
Build sbt/Scala client, servers (samples/client/petstore/scalaz)	3m 2s
Build sbt/Scala client, servers (samples/client/petstore/scala-pekko)	34s
Build sbt/Scala client, servers (samples/server/petstore/scala-lagom-server)	6m 36s
Build sbt/Scala client, servers (samples/server/petstore/scala-play-server)	5m 18s
Build sbt/Scala client, servers (samples/server/petstore/scala-akka-http-server)	2m 25s
Build sbt/Scala client, servers (samples/server/petstore/scala-pekko-http-server)	48s
Build sbt/Scala client, servers (samples/server/petstore/scalatra)	3m 8s
Build sbt/Scala client, servers (samples/server/petstore/scala-finch)	3m 18s
Build sbt/Scala client, servers (samples/server/petstore/scala-http4s-server)	52s
Build sbt/Scala client, servers (samples/server/petstore/scala-cask)	1m 8s
	27m 45s

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