fix: Use single value headers for HTTP API response (aws#377)

Parametrized the use of the single value headers for the response writer to support the v2 proxy schema for HTTP API

Author: sapessi

aws-serverless-java-container-springboot2/src/test/java/com/amazonaws/serverless/proxy/spring/ServletAppTest.java

@@ -23,8 +23,7 @@
 import java.util.Collection;
 import java.util.stream.Collectors;
 
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.*;
 
 @RunWith(Parameterized.class)
 public class ServletAppTest {
@@ -192,4 +191,18 @@ public void springExceptionMapping_throw404Ex_expectMappedTo404() {
         assertNotNull(resp);
         assertEquals(404, resp.getStatusCode());
     }
+
+    @Test
+    public void echoMessage_populatesSingleValueHeadersForHttpApiV2() {
+        AwsProxyRequestBuilder req = new AwsProxyRequestBuilder("/message", "POST")
+                .header(HttpHeaders.CONTENT_TYPE, "application/json;v=1")
+                .header(HttpHeaders.ACCEPT, "application/json;v=1")
+                .body(new MessageData("test message"));
+        AwsProxyResponse resp = handler.handleRequest(req, lambdaContext);
+        if ("HTTP_API".equals(type)) {
+            assertNotNull(resp.getHeaders());
+        } else {
+            assertNull(resp.getHeaders());
+        }
+    }
 }