FastbootAdapter requires maxAge to be set

[billdami]

Not sure if this was intentional, but it looks like the shoebox data expiration feature that was added via #369 that was added in the latest release causes the adapter to never use cached request data if a storefront.maxAge environment config value is not set. The current documentation makes it sound like this configuration is optional, which, imo, it probably should be. E.g.

isDateValid(createdString, maxAgeMinutes) {
    return !maxAgeMinutes || (new Date() - new Date(createdString)) / 1000 / 60 < maxAgeMinutes;
}

[st-h]

@billdami thanks. I thought I checked that it keeps the previous behaviour - but looking at the code now, I think you are right. I think your fix is reasonable. Could you please submit a PR?

[ryanto]

Released as 0.18.1 thanks to @xg-wang