title: JsonMappingException when serializing avro generated object to json
body: <p>I used avro-tools to generate java classes from avsc files, using:</p>
  
  <pre><code>java.exe -jar avro-tools-1.7.7.jar compile -string schema myfile.avsc 
  </code></pre>
  
  <p>Then I tried to serialize such objects to json by ObjectMapper,
  but always got a JsonMappingException saying "not an enum" or "not a union".
  In my test I create the generated object using it's builder or constructor.
  I got such exceptions for objects of different classes...</p>
  
  <p>Sample Code:</p>
  
  <pre><code>ObjectMapper serializer = new ObjectMapper(); // com.fasterxml.jackson.databind
  serializer.register(new JtsModule()); // com.bedatadriven.jackson.datatype.jts
  ...
  return serializer.writeValueAsBytes(avroConvertedObject); // =&gt; JsonMappingException
  </code></pre>
  
  <p>I also tried many configurations using: serializer.configure(...) but still failed.
  <strong>Versions:</strong> Java 1.8, jackson-datatype-jts 2.3, 
  jackson-core 2.6.5, jackson-databind 2.6.5, jackson-annotations 2.6.5</p>
  
  <p>Any suggestions?
  Thanks!</p>