Fix an PG overflow problem

In 4.2.0+, `#find_by` switched to using (cached!) prepared statements. As a side-
effect, doing out-of-range queries like `Topic.find_by(id: '9999999999999999999999999999')`
causes PG to complain instead of returning 0 rows. (Differece between sending
the literal 9999999999999999999 in a query and the quoted "9999999999999999999",
apparently.) Rescusing it for now, althought it might somehow be fixed upstream
in the future.
`Sprockets::Context` takes a `Pathname` for its third argument

Not sure what actually changed and why it was working before, but it now throws
a `NoMethodError` for `dirname` on `String` in `Context#resolve`
Use *_url instead of *_path to avoid a deprecation warning
HAX: check the `message` object, not the `MessageDelivery` object

See 669bf73 for background. It's probably better to rewrite these test without
using the internal `NullMail` class anyway.
Use the git repo for rails-dom-testing for now