title: Avro json decoder : ignore namespace
body: <p>i tried to use Apache Avro on project... and i've met some difficulties
  avro serialization/ deserialization work like a charm ... but  i get decoder exceptions.. like unknown union  branch blah-blah-blah... in case incomming json does't contain namepsace record ...
  e.g.  </p>
  
  <pre><code>"user":{"demo.avro.User":{"age":1000...  //that's ok
  
  "user":{"age":1000...  //org.apache.avro.AvroTypeException: Unknown union branch age
  </code></pre>
  
  <p>I cannot put object in default namespace... but it is important  to parse incoming json regardless it contains namespace node or not
  Could you help me to fix it</p>