[avro] Add support for Avro default values

[cowtowncoder]

(moved from FasterXML/jackson-dataformat-avro#34 by @turbospaces)

Right now RecordVisitor create Fields without default VALUE, which causes problem when the schema evaluates and client operates with OLD data http://ben-tech.blogspot.com/2013/05/avro-schema-evolution.html

I suggest to change Record visitor to:

@Override
public void optionalProperty(BeanProperty writer) throws JsonMappingException {
  Schema schema = schemaForWriter( writer );
  if ( !writer.getType().isPrimitive() ) {
      schema = AvroSchemaHelper.unionWithNull( schema );
  }
    _fields.add( new Schema.Field( writer.getName(), schema, null,
       writer.isRequired() ? null : Schema.parseJson( "null" ) ) );
}

[cowtowncoder]

Full support has been added now: this part is covered by full handling of default values during introspection.