Fix activerecord tests and empty collection handling

[tjdett]

This pull-request fixes the exist activerecord_provider tests and then adds a new test (and associated functionality) to ensure empty collections are handled in a sane fashion.

In the process:

• Database testing was switched to in-memory, resulting in a substantial performance improvement.
• Handling for sub-second date precision was added, as fixtures are all inserted within a second.
• All activerecord tests are now wrapped in transactions.

[tjdett]

@cbeer Is there anything in particular I should do to get my pull-requests reviewed?

[cbeer]

Thanks for the bump (and the patches); merged.