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;
 		}