Avoid UnsupportedEncodingException by design, replacing magic string "UTF-8" with StandardCharsets.UTF_8 #761

Job	Run time
Build sbt/Scala client, servers (samples/client/petstore/java/okhttp-gson)	26s
Build sbt/Scala client, servers (samples/client/petstore/scalaz)	2m 7s
Build sbt/Scala client, servers (samples/client/petstore/scala-pekko)	36s
Build sbt/Scala client, servers (samples/server/petstore/scala-lagom-server)	5m 49s
Build sbt/Scala client, servers (samples/server/petstore/scala-play-server)	4m 49s
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)	42s
Build sbt/Scala client, servers (samples/server/petstore/scalatra)	4m 54s
Build sbt/Scala client, servers (samples/server/petstore/scala-finch)	4m 15s
Build sbt/Scala client, servers (samples/server/petstore/scala-http4s-server)	49s
Build sbt/Scala client, servers (samples/server/petstore/scala-cask)	1m 7s
	27m 56s

Provide feedback