Fix for issue - https://issues.cloudera.org/browse/CDK-473

- CSV file header used to generate avro schema can only be Alphanumeric ( as per computer science definition).
- Additional check added to make sure that csv header should strictly follow Alphanumeric standards.
Can you wrap this line? I know it's annoying with a long string, but we try to keep lines to about 90 characters max. I'll add that to the list of things to add to our contributor guide.