When modifying the headers, split the value may cause unexpected bug

[spacewander]

What happened?

In

kuma/pkg/plugins/policies/meshhttproute/xds/filters/requestheadermodifier.go

Line 36 in 5c1f62d

for i, headerValue := range headerValues(set.Value) {

the header value is split by ,, and then added one by one.

In case like Set-Cookie: id=a3fWa; Expires=Wed, 21 Oct 2015 07:28:00 GMT, it will be added as:
Set-Cookie: id=a3fWa; Expires=Wed and
Set-Cookie: 21 Oct 2015 07:28:00 GMT

I wonder why not set the header directly without any splitting?

[michaelbeaumont]

This may be an oversight, particularly important for Set-Cookie, regarding directives from the gateway API docs at the time. I think Envoy will do the right thing here, we should change this.

[spacewander]

@michaelbeaumont
Would you assign this to me? I would submit a PR to change it a few days later.