filterGroups (search, label, values, groupLabel, customLabel) {
      return (groups) => groups.map((group) => {
        /* istanbul ignore else */
        if (!group[values]) {
          console.warn('Options passed to vue-multiselect do not contain groups, despite the config.');
          return []
        }
        const groupOptions = this.filterOptions(group[values], search, label, customLabel);

        return groupOptions.length
          ? {
              [groupLabel]: group[groupLabel], [values]: groupOptions
            }
          : []
      })
    }
@param {String} search
@param {String} label
@param {String} values
@param {String} groupLabel
@param {function} customLabel
@returns {function(*): *}