Fix ajv logging warnings when discriminators are combined with $ref's

[alonsohki]

This seems to be a bug when combining discriminators with references. In https://github.com/cdimascio/express-openapi-validator/blob/master/src/middlewares/parsers/schema.preprocessor.ts#L318 a properties field is added to the schema, even if empty. Then, ajv will detect that you have a object containing a $ref key along with an empty properties key, and trigger this piece of code that generates the said warning:

  if (it.schema.$ref && $refKeywords) {
    if (it.opts.extendRefs == 'fail') {
      throw new Error('$ref: validation keywords used in schema at path "' + it.errSchemaPath + '" (see option extendRefs)');
    } else if (it.opts.extendRefs !== true) {
      $refKeywords = false;
      it.logger.warn('$ref: keywords ignored in schema at path "' + it.errSchemaPath + '"');
    }
  }

[alonsohki]

Ping @cdimascio

[cdimascio]

@alonsohki thank you!

[cdimascio]

@all-contributors add @alonsohki for code

[allcontributors]

@cdimascio

I've put up a pull request to add @alonsohki! 🎉