use generics when implementing TupleToKafkaMapper in FieldNameBasedTuple..

- use generics when implementing TupleToKafkaMapper in FieldNameBasedTupleToKafkaMapper

It fixes unchecked assignment warning in the following piece of code:

```java
 public KafkaBolt kafkaBolt() {
        return new KafkaBolt<String, String>()
                .withTopicSelector(new DefaultTopicSelector(topicName))
                .withTupleToKafkaMapper(new FieldNameBasedTupleToKafkaMapper<String, String>());
    }
```