Make #fetch() *always* return nil if #exhausted?

[pilcrow]

Issue

Calling #fetch on an #exhausted? result set throws a Firebird "Request Synchronization Error," rather than repeatedly returning nil, as I'd expect from the documentation.

How to reproduce

See new unit test added in this pull request. Or, issue a SELECT, iterate with #each() or any Enumerable method on the ResultSet, and then call #fetch().

Proposed fixes

A few ways I can think of:

• Check result->exhausted before attempting a isc_dsql_fetch This is what this Pull Request implements.
• Ignore isc_req_sync errors thrown after an isc_dsql_fetch if we are exhausted. Implemented here: https://github.com/pilcrow/rubyfb/tree/exhausted-fetch
• Re-define the ResultSet's #fetch() method to return nil when we're exhausted.