Implementation of isAsyncStarted in proxy request was missing

Author: sapessi

aws-serverless-java-container-core/src/main/java/com/amazonaws/serverless/proxy/internal/servlet/AwsProxyHttpServletRequest.java

@@ -668,6 +668,11 @@ public boolean isAsyncSupported() {
         return true;
     }
 
+    @Override
+    public boolean isAsyncStarted() {
+        return asyncContext != null;
+    }
+
 
     @Override
     public AsyncContext startAsync()