=str,htp clean up build warnings

- explicitly provide Unit values and place parens around tuple creation
- remove structural type usage in TestUtils
- fix Java double-casts
- use unused Java values by asserting their non-nullness
- work around inability to place case class in trait (scripted test)

The remaining warnings about using private types in public methods are
bogus as reported in https://issues.scala-lang.org/browse/SI-9490.

akka-http-testkit/src/main/scala/akka/http/javadsl/testkit/RouteTest.scala

@@ -44,6 +44,7 @@ abstract class RouteTest extends AllDirectives {
 
     result.awaitResult(awaitDuration) match {
       case RouteResult.Complete(response) ⇒ createTestResponse(response)
+      case RouteResult.Rejected(ex)       ⇒ throw new AssertionError("got unexpected rejection: " + ex)
     }
   }