Force explicit use of `maybe` modifier in `map`.

There was a case when `map` would not call the predicate when the property was absent in an object, resulting in improper verification. For exemple, the use of `check.map( thing, { foo: verify.unemptyString });`  would never throw an error if `thing.foo` was not defined. Mapped predicate should be called regardless of the presence or absence of value in the verified object and the `maybe` modifier should be used to describe optional properties.