function createIsHttpErrorFunction (HttpError) {
	  return function isHttpError (val) {
	    if (!val || typeof val !== 'object') {
	      return false
	    }

	    if (val instanceof HttpError) {
	      return true
	    }

	    return val instanceof Error &&
	      typeof val.expose === 'boolean' &&
	      typeof val.statusCode === 'number' && val.status === val.statusCode
	  }
	}
Create function to test is a value is a HttpError.
@private