Improve CORS handling in AbstractSockJsService

After this change, AbstractSockJsService does not add CORS headers if
the response already contains an "Access-Control-Allow-Origin" header.
Essentially it backs off assuming CORS headers are handled centrally
e.g. through a Filter.

In order to support this, the ServletServerHttpResponse now returns an
instance of HttpHeaders that also provides access to headers already
present in the HttpServletResponse.

Issue: SPR-11443

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2014 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.
@@ -17,6 +17,7 @@
 package org.springframework.http.server;
 
 import java.nio.charset.Charset;
+import java.util.Arrays;
 import java.util.List;
 
 import org.junit.Before;
@@ -71,6 +72,19 @@ public class ServletServerHttpResponseTests {
 		assertEquals("Invalid Content-Type", "UTF-8", mockResponse.getCharacterEncoding());
 	}
 
+	@Test
+	public void getHeadersFromHttpServletResponse() {
+
+		String headerName = "Access-Control-Allow-Origin";
+		String headerValue = "localhost:8080";
+
+		this.mockResponse.addHeader(headerName, headerValue);
+		this.response = new ServletServerHttpResponse(this.mockResponse);
+
+		assertEquals(headerValue, this.response.getHeaders().getFirst(headerName));
+		assertEquals(Arrays.asList(headerValue), this.response.getHeaders().get(headerName));
+	}
+
 	@Test
 	public void getBody() throws Exception {
 		byte[] content = "Hello World".getBytes("UTF-8");