fixes #91 response headers are not returned if there is an error

Author: stevehu

src/main/java/com/networknt/aws/lambda/LightLambdaExchange.java

@@ -216,9 +216,9 @@ public APIGatewayProxyResponseEvent getFinalizedResponse(boolean fromListener) {
         }
         /*
          * Check for failures first because a failed request could mean
-         * that we never set the response in the first place.
+         * that we never set the response in the first place. If response is not null, don't bother.
          */
-        if (stateHasAnyFlags(FLAG_REQUEST_HAS_FAILURE | FLAG_RESPONSE_HAS_FAILURE)) {
+        if (stateHasAnyFlags(FLAG_REQUEST_HAS_FAILURE | FLAG_RESPONSE_HAS_FAILURE) && response == null) {
             LOG.error("Exchange has an error, returning middleware status.");
             this.state |= FLAG_RESPONSE_DONE;
             return ExceptionUtil.convert(this.executor.getChainResults());

src/main/java/com/networknt/aws/lambda/handler/middleware/ExceptionUtil.java

@@ -1,12 +1,9 @@
 package com.networknt.aws.lambda.handler.middleware;
 import com.amazonaws.services.lambda.runtime.events.APIGatewayProxyResponseEvent;
-import com.fasterxml.jackson.core.JsonProcessingException;
-import com.fasterxml.jackson.databind.ObjectMapper;
 import com.networknt.aws.lambda.utility.HeaderKey;
 import com.networknt.aws.lambda.utility.HeaderValue;
 import com.networknt.status.Status;
 
-import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
 
@@ -16,12 +13,6 @@
  *
  */
 public class ExceptionUtil {
-
-    private static final String DATA_KEY = "data";
-    private static final String NOTIFICATIONS_KEY = "notifications";
-
-    private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
-
     /**
      * Convert the middleware results to APIGatewayProxyResponseEvent
      *