ResponseEntity's headers(HttpHeaders) accepts null value

Issue: SPR-12792

spring-web/src/test/java/org/springframework/http/ResponseEntityTests.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.
@@ -27,6 +27,7 @@ import static org.junit.Assert.*;
 /**
  * @author Arjen Poutsma
  * @author Marcel Overdijk
+ * @author Kazuki Shimizu
  */
 public class ResponseEntityTests {
 
@@ -161,4 +162,31 @@ public class ResponseEntityTests {
 		assertNull(responseEntity.getBody());
 	}
 
+	@Test
+	public void headersCopy(){
+		HttpHeaders customHeaders = new HttpHeaders();
+		customHeaders.set("X-CustomHeader", "vale");
+
+		ResponseEntity<Void> responseEntity = ResponseEntity.ok().headers(customHeaders).build();
+		HttpHeaders responseHeaders = responseEntity.getHeaders();
+
+		assertEquals(HttpStatus.OK, responseEntity.getStatusCode());
+		assertEquals(1, responseHeaders.size());
+		assertEquals(1, responseHeaders.get("X-CustomHeader").size());
+		assertEquals("vale", responseHeaders.getFirst("X-CustomHeader"));
+
+	}
+
+	@Test  // SPR-12792
+	public void headersCopyWithEmptyAndNull(){
+		ResponseEntity<Void> responseEntityWithEmptyHeaders =
+				ResponseEntity.ok().headers(new HttpHeaders()).build();
+		ResponseEntity<Void> responseEntityWithNullHeaders =
+				ResponseEntity.ok().headers(null).build();
+
+		assertEquals(HttpStatus.OK, responseEntityWithEmptyHeaders.getStatusCode());
+		assertTrue(responseEntityWithEmptyHeaders.getHeaders().isEmpty());
+		assertEquals(responseEntityWithEmptyHeaders.toString(), responseEntityWithNullHeaders.toString());
+	}
+
 }