Ensure RedirectModel is initialized

This commit fixes an old bug in ModelAndViewContainer where getModel
returns a new ModelMap instance that isn't saved and re-used.

Issue: SPR-14045

spring-web/src/main/java/org/springframework/web/method/support/ModelAndViewContainer.java

@@ -134,7 +134,10 @@ public class ModelAndViewContainer {
 			return this.defaultModel;
 		}
 		else {
-			return (this.redirectModel != null) ? this.redirectModel : new ModelMap();
+			if (this.redirectModel == null) {
+				this.redirectModel = new ModelMap();
+			}
+			return this.redirectModel;
 		}
 	}