=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/src/main/scala/akka/http/impl/server/RouteImplementation.scala

@@ -151,7 +151,7 @@ private[http] object RouteImplementation extends Directives with server.RouteCon
         (requestValToDirective(dyn.value1) & requestValToDirective(dyn.value2))(runToRoute)
 
       case o: OpaqueRoute ⇒ (ctx ⇒ o.handle(new RequestContextImpl(ctx)).asInstanceOf[RouteResultImpl].underlying)
-      case p: Product     ⇒ extractExecutionContext { implicit ec ⇒ complete(500, s"Not implemented: ${p.productPrefix}") }
+      case p: Product     ⇒ extractExecutionContext { implicit ec ⇒ complete((500, s"Not implemented: ${p.productPrefix}")) }
     }
   }
   def pathMatcherDirective[T](matchers: immutable.Seq[PathMatcher[_]],