ClassCastExeption when reading Avro files using specific records with org.apache.avro.util.Utf8 fields

Reading Avro files using Avro-generated classes with org.apache.avro.util.Utf8 fields fails with an exception:
```

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to org.apache.avro.util.Utf8
  
 at com.example.avro.AvroRecord.put(AvroRecord.java:129)
    at org.apache.avro.generic.GenericData.setField(GenericData.java:690)
  
 at org.apache.avro.reflect.ReflectData.setField(ReflectData.java:135)
    at org.apache.avro.reflect.ReflectData.setField(ReflectData.java:128)
  
 at org.apache.avro.specific.SpecificDatumReader.readField(SpecificDatumReader.java:119)
    at org.apache.avro.reflect.ReflectDatumReader.readField(ReflectDatumReader.java:310)
  
 at org.apache.avro.generic.GenericDatumReader.readRecord(GenericDatumReader.java:222)
    at org.apache.avro.generic.GenericDatumReader.readWithoutConversion(GenericDatumReader.java:175)
  
 at org.apache.avro.generic.GenericDatumReader.read(GenericDatumReader.java:153)
    at org.apache.avro.generic.GenericDatumReader.read(GenericDatumReader.java:145)
  
 at org.apache.beam.sdk.io.AvroSource$AvroBlock.readNextRecord(AvroSource.java:577)
    at org.apache.beam.sdk.io.BlockBasedSource$BlockBasedReader.readNextRecord(BlockBasedSource.java:223)
  
 at org.apache.beam.sdk.io.FileBasedSource$FileBasedReader.advanceImpl(FileBasedSource.java:473)
  
 at org.apache.beam.sdk.io.FileBasedSource$FileBasedReader.startImpl(FileBasedSource.java:468)
    at
org.apache.beam.sdk.io.OffsetBasedSource$OffsetBasedReader.start(OffsetBasedSource.java:261)
    at
org.apache.beam.runners.direct.BoundedReadEvaluatorFactory$BoundedReadEvaluator.processElement(BoundedReadEvaluatorFactory.java:141)
  
 at org.apache.beam.runners.direct.DirectTransformExecutor.processElements(DirectTransformExecutor.java:161)
  
 at org.apache.beam.runners.direct.DirectTransformExecutor.run(DirectTransformExecutor.java:125)
  
 at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
  
 at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
  
 at java.lang.Thread.run(Thread.java:748)

```

Imported from Jira [BEAM-4064](https://issues.apache.org/jira/browse/BEAM-4064). Original Jira may contain additional context.
Reported by: przemekd.