handle ignore_status_codes defined as strings

This should solve the problem with `NEW_RELIC_ERROR_COLLECTOR_IGNORE_ERROR_CODES` not working as expected.

For example, if you want to ignore in your 401 errors, and 404 errors, you can set:

    NEW_RELIC_ERROR_COLLECTOR_IGNORE_ERROR_CODES=404,401

and it should work with that change.