Numeric is not a dry-types built-in type, fix coercion/validation

[Vasfed]

In 1.3+ if someone had a Numeric type parameter which worked in <1.3 - any valid non-blank value will silently fail validation

params { optional :price, type: Numeric }

This happens because dry-types does not contain built-in type for DryTypes::SomeScope::Numeric and const_get by default searches ancestors, thus finding ::Numeric.
Runtime error undefined method `[]' for Numeric:Class is treated as a validation error and leads to unexpected behavior.

This PR adds mapping for Numeric that fixes it, also fallback to const_get(.., false) will make similar issues easier to debug.

[dblock]

Right on. The change to not lookup inherited types in const_get seems to break a bunch of other stuff, though.

 30) Grape::Endpoint#declared when the param is missing and include_missing=false sets nested objects to be nil
      Failure/Error: MAPPING.fetch(type) { scope.const_get(type.name, false) }
      NameError:
        uninitialized constant #<Module:0x00005598ebeb08a0>::Set

[Vasfed]

Yep, reverted the const_get change, it's not that important for the fix

[dblock]

Can we just revert this part of the change and open a new issue on GH please?

[Vasfed]

Done

[dblock]

Thanks! Merged.