have the `FakeRecord` mimic ARs `column_names`

AR exposes its columns using the class reader `column_names`.  This
mimics the behavior of that reader in MR fake records.

It uses the configured `attributes` as the source for the column
names, where a real AR record would use the data schema.
Merge pull request #31 from redding/kr-colnames

have the `FakeRecord` mimic ARs `column_names`