fix: headers with null values removed from response (#1523)

Co-authored-by: Emil Kruczek <emil.kruczek@external.nexigroup.com>

Author: ekruczek

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

@@ -496,9 +496,12 @@ private void setHeader(String key, String value, boolean overwrite) {
             values = new ArrayList<>();
         }
 
-        values.add(encodedValue);
-
-        headers.put(encodedKey, values);
+        if (value == null && overwrite) {
+            headers.remove(encodedKey);
+        } else if (value != null) {
+            values.add(encodedValue);
+            headers.put(encodedKey, values);
+        }
     }
 
     private boolean canSetHeader() {

aws-serverless-java-container-core/src/test/java/com/amazonaws/serverless/proxy/internal/servlet/AwsHttpServletResponseTest.java

@@ -173,6 +173,16 @@ void responseHeaders_getAwsResponseHeaders_expectLatestHeader() {
         assertEquals("application/xml", awsResp.getFirst(HttpHeaders.CONTENT_TYPE));
     }
 
+    @Test
+    void responseHeaders_setHeaderWithNullValue_expectHeaderRemoved() {
+        AwsHttpServletResponse resp = new AwsHttpServletResponse(null, null);
+        resp.setHeader(HttpHeaders.CONTENT_DISPOSITION, "inline");
+        resp.setHeader(HttpHeaders.CONTENT_DISPOSITION, null);
+
+        Headers awsResp = resp.getAwsResponseHeaders();
+        assertEquals(0, awsResp.size());
+    }
+
     @Test
     void responseHeaders_getAwsResponseHeaders_expectedMultpleCookieHeaders() {
         AwsHttpServletResponse resp = new AwsHttpServletResponse(null, null);