Add support for Sequel datasets

[martijnbleeker]

Sequel dataset do not support the to_ary implementation. To add support, check if objects is a list.

Note: nil and Hash both implement to_a but aren't lists themselves.

Fixes #188

Please let me know if this fix is ok. Do you think only checking nil and Hash is enough?

[dblock]

You also need tests for this to be merged and a CHANGELOG entry, please.