Fix default value for Expires header

[thornbill]

Changes
Changes the default value of the Expires HTTP header to be "0" instead of "-1". The RFC specifically mentions using "0" to represent values that have already expired.

The Java apiclient uses Gson for serializing the response and it logs warnings for the use of "-1".

Issues
N/A

[LogicalPhallacy]

Seems like it should have raised a problem before. Fix looks good.

[cvium]

The spec and what's actually implemented can be two different things. Eg. https://support.microsoft.com/en-us/help/234067/how-to-prevent-caching-in-internet-explorer and https://stackoverflow.com/a/11367872

And if you read the spec:

A cache recipient MUST interpret invalid date formats, especially the
value "0", as representing a time in the past (i.e., "already
expired").

It simply states that any invalid date value must be interpreted as "already expired".

If we want to be truly "correct", the default value should be changed to a date in the past.

[JustAMan]

the default value should be changed to a date in the past.

Can we just set the date to "Unix start" (01-01-1970)?

[joshuaboniface]

I disagree, the spec seems to indicate that 0 is the preferred way to specify the invalid date. Which makes sense, it's Unix start minus 1. I'm not in favour of hacks to support Microsoft's abomination known as IE. It's dead anyways.

[cvium]

If you want to be really pedantic about the wording in the spec, you're actually supposed to set the Expires header to the same value as the Date header.

[joshuaboniface]

I don't really care either way, 0 is an invalid date in the past so it fits what's required. But if the change works it works, isn't a hack, and seems to follow the spec. Putting 1970-01-01 00:00:00 is a hack to me.

[joshuaboniface]

So how do we want to handle this @thornbill @cvium?

[thornbill]

I don’t have a strong opinion either way, but anything beyond a basic string replacement will be exceeding my C# abilities. 😉

[joshuaboniface]

Let's just make it work however works best for you @cvium I'd like this to be in the release but don't want to delay it too much longer.

[cvium]

It's fine. I just don't agree that -1 is a hack as it's valid according to the spec. I haven't been able to find any information on whether this is still an issue in Edge, so let's just give it a shot...