Assign 'tuple' to 'stringTuple' in DefaultRedisSerializationContextBuilder.string().

Currently, the builder method incorrectly assigns 'tuple' SerializationPair<String> argument to 'hashValueTuple'.

Closes #2651
Original pull request: #2652

src/main/java/org/springframework/data/redis/serializer/DefaultRedisSerializationContext.java

@@ -24,6 +24,7 @@ import org.springframework.util.Assert;
  * @author Mark Paluch
  * @author Christoph Strobl
  * @author Shyngys Sapraliyev
+ * @author Zhou KQ
  * @since 2.0
  */
 class DefaultRedisSerializationContext<K, V> implements RedisSerializationContext<K, V> {
@@ -127,7 +128,7 @@ class DefaultRedisSerializationContext<K, V> implements RedisSerializationContex
 
 			Assert.notNull(tuple, "SerializationPair must not be null");
 
-			this.hashValueTuple = tuple;
+			this.stringTuple = tuple;
 			return this;
 		}