Fix for PackedAvroSource issue

This allows a JobTest formatted like below to be supported:

```scala
JobTest("my.main.class")
   .arg("input", "myInput")
   .arg("output", "myOutput")
   .source(...)
   .sink[MyAvroObject](PackedAvroSource[MyAvroObject]("myOutput")) { 
      op => println(op.toList)
   }
   .runHadoop
   .finish
```