=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-tests-java8/src/test/java/docs/http/javadsl/server/HandlerExampleDocTest.java

@@ -11,13 +11,10 @@ import akka.http.javadsl.server.values.Parameters;
 import akka.http.javadsl.server.values.PathMatchers;
 import akka.http.javadsl.testkit.JUnitRouteTest;
 import akka.http.javadsl.testkit.TestRoute;
-import akka.japi.function.Function;
 import org.junit.Test;
 import scala.concurrent.ExecutionContext;
 import scala.concurrent.Future;
 
-import java.util.concurrent.Callable;
-
 public class HandlerExampleDocTest extends JUnitRouteTest {
     @Test
     public void testSimpleHandler() {
@@ -25,6 +22,7 @@ public class HandlerExampleDocTest extends JUnitRouteTest {
         class TestHandler extends akka.http.javadsl.server.AllDirectives {
             //#simple-handler
             Handler handler = new Handler() {
+                static final long serialVersionUID = 1L;
                 @Override
                 public RouteResult apply(RequestContext ctx) {
                     return ctx.complete("This was a " + ctx.request().method().value()  +
@@ -75,6 +73,7 @@ public class HandlerExampleDocTest extends JUnitRouteTest {
             //#handler2
             final Handler2<Integer, Integer> multiply =
                 new Handler2<Integer, Integer>() {
+                    static final long serialVersionUID = 1L;
                     @Override
                     public RouteResult apply(RequestContext ctx, Integer x, Integer y) {
                         int result = x * y;