Raise when calling connection on a sharded model without a shard

[pschambacher]

/cc @bquorning @grosser @kassio @craig-day

When calling Model.connection on a sharded model when we're not on a shard returns a connection to the unsharded database. This PR fixes the issue by raising when trying to call connection without specifying a shard.

fyi @jacobat

[jacobat]

How does this play with #130?

[pschambacher]

@jacobat I would assume your PR would be fine. Our issue in Classic is some code paths relying on default_shard to be set, but it's actually nil. Removing default_shard would probably raise the same way it does here.