combine multiple headers using comma

[Lidbetter]

This addresses issue #1190 (Multiple Set-Cookie headers get aggregated)

https://tools.ietf.org/html/rfc2616#section-4.2

It MUST be possible to combine the multiple header fields into one
"field-name: field-value" pair, without changing the semantics of the
message, by appending each subsequent field-value to the first, each
separated by a comma.

I searched newer RFCs and as far as I can tell the above still holds true

One thing to note, is this:
https://tools.ietf.org/html/rfc6265#section-3

Origin servers SHOULD NOT fold multiple Set-Cookie header fields into
a single header field. The usual mechanism for folding HTTP headers
fields (i.e., as defined in [RFC2616]) might change the semantics of
the Set-Cookie header field because the %x2C (",") character is used
by Set-Cookie in a way that conflicts with such folding.

With that in mind (and some individual header spesific things outlined in other RFCs) it may be better to rework how these headers flow through the application so that multiple headers do not need to be serialized.

For now joining with a comma fixes the set-cookie issue and seems to be more correct than joining with a space

[XVincentX]

Thanks a lot for the PR. I've put a couple of comments but we're definitely on the way to fix this!

[XVincentX]

This is probably the good change — but I'd say we only need to apply this on the Set-Cookie header instead of doing it for all of them. mapValues is likely offering an overload to also get the key and react accordingly

[Lidbetter]

The reason I didn't initially only special case set-cookie was because of the wording in rfc2616, as far as I can tell joining with a comma is right thing to do by default - a space would be the exception to the general rule.

I could be wrong, do you have a link to more/newer info about the correct way to combine multiple header fields into one?

[XVincentX]

I need to take a deeper look into; the reason is that it seems like such rule applies only for "state" headers that is Cookies, Caching and some other that now I do not remember.

You can leave it as it is. I'll check it out later or tomorrow and then get back to you.

[XVincentX]

We'll also have to write a more specific test to make sure it only happens only under specific conditions.

[XVincentX]

Ok I've reviewed the entire thing now. While it won't fix a probable issue with Set-Cookie with commas, it should still do the job for now. Thanks