utils-tests: add unit tests for prepareValue
utils: reorganize prepareValue conditional for clarity

Prefer positive tests; group tests for specific objects.
properly prepare complex arrays

`arrayString` duplicated too much of `prepareValue`'s logic, and so
didn't receive bugfixes for handling dates with timestamps. Defer to
`prepareValue` whenever possible.

This change enforces double-quote escaping of all array elements,
regardless of whether escaping is necessary. This has the side-effect of
properly escaping JSON arrays.
allow type-coercion overrides for custom objects

Attempt to call a `toPostgres` method on objects passed as query values
before converting them to JSON. This allows custom types to convert
themselves to the appropriate PostgreSQL literal.

This strategy is fully backwards-compatible and uses the same pattern as
the `toJSON` override.