Use String's bytesize instead of size, because we're talking about byte length, not string length. This resolves a problem on Ruby 1.9 with multi-byte characters.
https://github.com/zdennis/activerecord-import/issues#issue/6
https://github.com/zdennis/activerecord-import/issues#issue/12
* removed assertions on #num_inserts, that is db-specific and is handled in details by abstract_adapter_test.