Vex.Validator.Source.check(validator) only verifies `validate/2`

[jfornoff]

I believe that the verification logic for validator modules is missing a case for validators that use validate/3 instead of validate/2.

Technically, how I perceive the logic to be working is:

• validate/3 is called at runtime
• use Vex.Validator defines a default validate/3 that delegates to validate/2, discarding the context attribute
• When looking up validators by name (done in Vex.validator/1), Vex.Validator.Sources encodes what is considered a complete validator module: exporting validate/2

However, this verification assumes that validate/3 is the not overridden and the default implementation delegates to validate/2, otherwise it would be perfectly reasonable to not have validate/2 and instead just defining validate/3