=akka-http remove unnecessary copying of entity (#2522)

Just a small improvement which I noticed while skimming the code.

The HttpEntity call made on this line already makes the content type be `text/plain` (UTF-8), the additional call made here created an entire copy of the entity object for each request and did not really have to do that since it is the right content type already. See here: https://github.com/akka/akka-http/blob/master/akka-http-core/src/main/scala/akka/http/scaladsl/model/HttpEntity.scala#L253
https://github.com/akka/akka-http/blob/master/akka-http-core/src/main/scala/akka/http/scaladsl/model/HttpEntity.scala#L253

frameworks/Scala/akka-http/src/main/scala/com/typesafe/akka/http/benchmark/handlers/PlaintextHandler.scala

@@ -14,5 +14,5 @@ class PlaintextHandler(components: {
    }
  }

  def response = HttpResponse(StatusCodes.OK, entity = HttpEntity("Hello, World!").withContentType(`text/plain`.withCharset(`UTF-8`)))
  def response = HttpResponse(StatusCodes.OK, entity = HttpEntity("Hello, World!"))
}