Relax generic type checks when `@Argument` binding to a map with an undeclared value type

[rstoyanchev]

Currently we reject @Argument Map<String, ?> but that's arguably too strict. We could treat it the same way as Map<String, Object>, i.e. as intending to access the raw value. It's the best we can do from the given inputs, and no need to require declaring the value type as Object.

Originally discussed under #548.