Fixes #1209 - IllegalStateException when HTTP/2 push is disabled.

Aligned the behavior with Servlet 4.0, returning null if push is
not supported or disabled by the client.

jetty-server/src/main/java/org/eclipse/jetty/server/Request.java

@@ -285,7 +285,7 @@ public class Request implements HttpServletRequest
     public PushBuilder getPushBuilder()
     {
         if (!isPushSupported())
-            throw new IllegalStateException(String.format("%s,push=%b,channel=%s", this, isPush(), getHttpChannel()));
+            return null;
 
         HttpFields fields = new HttpFields(getHttpFields().size()+5);
         boolean conditional=false;