validateMethods(methods) {
        let invalidMethod;
        function isValidMethod(method) {
            const acceptedValues = ['GET', 'PUT', 'HEAD', 'POST', 'DELETE'];
            if (acceptedValues.indexOf(method) !== -1) {
                return true;
            }
            invalidMethod = method;
            return false;
        }
        if (!methods.every(isValidMethod)) {
            const errMsg = 'Found unsupported HTTP method in CORS config. ' +
            `Unsupported method is "${invalidMethod}"`;
            return errorInstances.InvalidRequest.customizeDescription(errMsg);
        }
        return true;
    }
_validator.validateMethods - check values of AllowedMethod's
@param {string[]} methods - array of AllowedMethod's
@return {(Error|true)} - Arsenal error on failure, true on success