Don't check joi alternative for "is"

[onufriievkyrylo]

We shouldn't be forced to use "is" in schemas because we could use schema based condition with our alternatives statement so please remove behavior with throwing error when (!match.is)

[benrei]

This package uses the 'joi-to-json-schema' to convert the Joi schemas. It is in that package the 'throw error when (!match.is)' is located.
I see there's already an open issue. I will update the dependency when it's fixed.