title: How to aggregate many marshalled (json) objects to one file
body: <p>I created a route to buffer/store marshaled objects (json) into files. This route (and the other route to read the buffer) work fine. 
  storing in buffer: </p>
  
  <pre><code>from(DIRECT_IN).marshal().json().marshal().gzip().to(fileTarget()); 
  </code></pre>
  
  <p>reading from buffer: </p>
  
  <pre><code>from(fileTarget()).unmarshal().gzip().unmarshal().json().to("mock:a")
  </code></pre>
  
  <p>To reduce i/o i want to aggregate many exchanges in one file. I tried to just aggregate after json and before it so i added this after json() or from(...): </p>
  
  <pre><code>.aggregate(constant(true)).completionSize(20).completionTimeout(1000).groupExchanges()
  </code></pre>
  
  <p>In both cases i get conversion exceptions. How to do it correctly? I would prefer a way without custom aggregator. And it would be nice if just many exchanges/object are aggregated in one json (as list of objects) or in one text file - one json object per line. </p>
  
  <p>Thanks in advance. </p>