message: Fix the validation for allowed primary key types
  Not all indexable property types are actually allowed as PKs.
diff: @@ -180,7 +180,7 @@ static void validate_property(Schema const& schema,
  // check primary keys
  if (prop.is_primary) {
  - if (!prop.type_is_indexable()) {
  + if (prop.type != PropertyType::Int && prop.type != PropertyType::String) {
  exceptions.emplace_back("Property '%1.%2' of type '%3' cannot be made the primary key.",
  object_name, prop.name, string_for_property_type(prop.type));
  }
  