Quote ETags set with ResponseEntity builder API

Prior to this change, trying to set an unquoted ETag with
`ResponseEntity`'s API would throw an `IllegalArgumentException`.

This commit automatically quotes ETag values set using ResponseEntity.

Issue: SPR-13378

spring-web/src/main/java/org/springframework/http/ResponseEntity.java

@@ -421,6 +421,14 @@ public class ResponseEntity<T> extends HttpEntity<T> {
 
 		@Override
 		public BodyBuilder eTag(String eTag) {
+			if (eTag != null) {
+				if(!eTag.startsWith("\"") && !eTag.startsWith("W/\"")) {
+					eTag = "\"" + eTag;
+				}
+				if(!eTag.endsWith("\"")) {
+					eTag = eTag + "\"";
+				}
+			}
 			this.headers.setETag(eTag);
 			return this;
 		}

spring-web/src/test/java/org/springframework/http/ResponseEntityTests.java

@@ -134,14 +134,12 @@ public class ResponseEntityTests {
 
 	@Test
 	public void headers() throws URISyntaxException {
-		String eTag = "\"foo\"";
 		URI location = new URI("location");
 		long contentLength = 67890;
 		MediaType contentType = MediaType.TEXT_PLAIN;
 
 		ResponseEntity<Void> responseEntity = ResponseEntity.ok().
 				allow(HttpMethod.GET).
-				eTag(eTag).
 				lastModified(12345L).
 				location(location).
 				contentLength(contentLength).
@@ -153,7 +151,6 @@ public class ResponseEntityTests {
 		HttpHeaders responseHeaders = responseEntity.getHeaders();
 
 		assertEquals("GET", responseHeaders.getFirst("Allow"));
-		assertEquals(eTag, responseHeaders.getFirst("ETag"));
 		assertEquals("Thu, 01 Jan 1970 00:00:12 GMT",
 				responseHeaders.getFirst("Last-Modified"));
 		assertEquals(location.toASCIIString(),
@@ -164,6 +161,19 @@ public class ResponseEntityTests {
 		assertNull(responseEntity.getBody());
 	}
 
+	@Test
+	public void Etagheader() throws URISyntaxException {
+
+		ResponseEntity<Void> responseEntity = ResponseEntity.ok().eTag("\"foo\"").build();
+		assertEquals("\"foo\"", responseEntity.getHeaders().getETag());
+
+		responseEntity = ResponseEntity.ok().eTag("foo").build();
+		assertEquals("\"foo\"", responseEntity.getHeaders().getETag());
+
+		responseEntity = ResponseEntity.ok().eTag("W/\"foo\"").build();
+		assertEquals("W/\"foo\"", responseEntity.getHeaders().getETag());
+	}
+
 	@Test
 	public void headersCopy() {
 		HttpHeaders customHeaders = new HttpHeaders();