Add containsKey to ServletResponseHttpHeaders

Issue: SPR-13668

spring-web/src/main/java/org/springframework/http/server/ServletServerHttpResponse.java

@@ -137,6 +137,11 @@ public class ServletServerHttpResponse implements ServerHttpResponse {
 
 		private static final long serialVersionUID = 3410708522401046302L;
 
+		@Override
+		public boolean containsKey(Object key) {
+			return (super.containsKey(key) || (get(key) != null));
+		}
+
 		@Override
 		public String getFirst(String headerName) {
 			String value = servletResponse.getHeader(headerName);

spring-web/src/test/java/org/springframework/http/server/ServletServerHttpResponseTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,7 +17,7 @@
 package org.springframework.http.server;
 
 import java.nio.charset.Charset;
-import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 
 import org.junit.Before;
@@ -29,10 +29,13 @@ import org.springframework.http.MediaType;
 import org.springframework.mock.web.test.MockHttpServletResponse;
 import org.springframework.util.FileCopyUtils;
 
-import static org.junit.Assert.*;
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
 
 /**
  * @author Arjen Poutsma
+ * @author Rossen Stoyanchev
  */
 public class ServletServerHttpResponseTests {
 
@@ -40,12 +43,14 @@ public class ServletServerHttpResponseTests {
 
 	private MockHttpServletResponse mockResponse;
 
+
 	@Before
 	public void create() throws Exception {
 		mockResponse = new MockHttpServletResponse();
 		response = new ServletServerHttpResponse(mockResponse);
 	}
 
+
 	@Test
 	public void setStatusCode() throws Exception {
 		response.setStatusCode(HttpStatus.NOT_FOUND);
@@ -73,7 +78,7 @@ public class ServletServerHttpResponseTests {
 	}
 
 	@Test
-	public void getHeadersFromHttpServletResponse() {
+	public void preExistingHeadersFromHttpServletResponse() {
 
 		String headerName = "Access-Control-Allow-Origin";
 		String headerValue = "localhost:8080";
@@ -82,7 +87,8 @@ public class ServletServerHttpResponseTests {
 		this.response = new ServletServerHttpResponse(this.mockResponse);
 
 		assertEquals(headerValue, this.response.getHeaders().getFirst(headerName));
-		assertEquals(Arrays.asList(headerValue), this.response.getHeaders().get(headerName));
+		assertEquals(Collections.singletonList(headerValue), this.response.getHeaders().get(headerName));
+		assertTrue(this.response.getHeaders().containsKey(headerName));
 	}
 
 	@Test