title: string in double quotes after a map operation in Kafka streams
body: <p>I use Kafka streams DSL and map to convert a <code>KStream&lt;String, JsonNode&gt;</code> to <code>KStream&lt;String, String&gt;</code>.</p>
  
  <p>In <code>ValueMapper</code> function, I simple return <code>new ValueMapper("key", "some constant string")</code>, but where the value is sent back to Kafka use <code>KStream.to("some topic")</code>, I got the result is being added double quotes.</p>
  
  <p>my code is like this:</p>
  
  <pre><code>KStream&lt;String, JsonNode&gt; views = builder.stream("fromTopic");
  views.map(new ValueKMapper()).to("toTopic");
  </code></pre>
  
  <p>and <code>ValueMapper</code> simply implements <code>KeyValueMapper</code> and the code of <code>apply()</code> is:</p>
  
  <pre><code>public KeyValue&lt;String, String&gt; apply(String key, JsonNode value) 
  {
      return new KeyValue&lt;String,String&gt;("a", "hello");
  }
  </code></pre>
  
  <p>and then when I consume the <code>toTopic</code>, I got the <code>""hello""</code>, with quotes added.</p>
  
  <p>Maybe it's a bug of Kafka streams?</p>