_filterUpgrades(upgrades) {
    const filteredUpgrades = [];
    for (let i = 0; i < upgrades.length; i++) {
      if (~this.transports.indexOf(upgrades[i]))
        filteredUpgrades.push(upgrades[i]);
    }
    return filteredUpgrades;
  }
Filters upgrades, returning only those matching client transports.

@param {Array} upgrades - server upgrades
@private