=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-docs-dev/rst/java/code/docs/http/javadsl/HttpClientExampleDocTest.java

@@ -16,6 +16,7 @@ import akka.http.javadsl.model.*;
 import akka.http.javadsl.Http;
 import scala.util.Try;
 
+@SuppressWarnings("unused")
 public class HttpClientExampleDocTest {
 
     // compile only test

akka-docs-dev/rst/java/code/docs/http/javadsl/ModelDocTest.java

@@ -13,6 +13,7 @@ import akka.http.javadsl.model.*;
 import akka.http.javadsl.model.headers.*;
 //#import-model
 
+@SuppressWarnings("unused")
 public class ModelDocTest {
     @Test
     public void testConstructRequest() {
@@ -78,7 +79,7 @@ public class ModelDocTest {
     //#headers
 
     // a method that extracts basic HTTP credentials from a request
-    private Option<BasicHttpCredentials> getCredentialsOfRequest(HttpRequest request) {
+	private Option<BasicHttpCredentials> getCredentialsOfRequest(HttpRequest request) {
         Option<Authorization> auth = request.getHeader(Authorization.class);
         if (auth.isDefined() && auth.get().credentials() instanceof BasicHttpCredentials)
             return Option.some((BasicHttpCredentials) auth.get().credentials());

akka-docs-dev/rst/java/code/docs/http/javadsl/server/HttpServerExampleDocTest.java

@@ -10,16 +10,12 @@ import akka.http.javadsl.Http;
 import akka.http.javadsl.IncomingConnection;
 import akka.http.javadsl.ServerBinding;
 import akka.http.javadsl.model.*;
-import akka.http.javadsl.model.ws.Websocket;
 import akka.japi.function.Function;
 import akka.japi.function.Procedure;
 import akka.stream.ActorMaterializer;
 import akka.stream.Materializer;
-import akka.stream.javadsl.Flow;
 import akka.stream.javadsl.Sink;
 import akka.stream.javadsl.Source;
-import org.junit.Test;
-import scala.Function1;
 import scala.concurrent.Await;
 import scala.concurrent.Future;
 import scala.concurrent.duration.FiniteDuration;
@@ -29,7 +25,7 @@ import java.io.InputStreamReader;
 import java.util.concurrent.TimeUnit;
 
 public class HttpServerExampleDocTest {
-    public static void bindingExample() {
+    public static void bindingExample() throws Exception {
         //#binding-example
         ActorSystem system = ActorSystem.create();
         Materializer materializer = ActorMaterializer.create(system);
@@ -47,6 +43,7 @@ public class HttpServerExampleDocTest {
                     }
                 })).run(materializer);
         //#binding-example
+        Await.result(serverBindingFuture, new FiniteDuration(3, TimeUnit.SECONDS));
     }
     public static void fullServerExample() throws Exception {
         //#full-server-example