HeaderValueHolder.toString() exposes underlying list of header values

Issue: SPR-15375
(cherry picked from commit 15bff8f)

spring-test/src/main/java/org/springframework/mock/web/HeaderValueHolder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -72,7 +72,12 @@ class HeaderValueHolder {
 	}
 
 	public String getStringValue() {
-		return (!this.values.isEmpty() ? this.values.get(0).toString() : null);
+		return (!this.values.isEmpty() ? String.valueOf(this.values.get(0)) : null);
+	}
+
+	@Override
+	public String toString() {
+		return this.values.toString();
 	}
 
 

spring-web/src/test/java/org/springframework/mock/web/test/HeaderValueHolder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -72,7 +72,12 @@ class HeaderValueHolder {
 	}
 
 	public String getStringValue() {
-		return (!this.values.isEmpty() ? this.values.get(0).toString() : null);
+		return (!this.values.isEmpty() ? String.valueOf(this.values.get(0)) : null);
+	}
+
+	@Override
+	public String toString() {
+		return this.values.toString();
 	}