http2: pass incoming set-cookie header as array
Incoming set-cookie headers should be passed to user as array like in http module - even for single occurrence.
Besides improving compatibility between http and http2 it avoids the need to check if the type is an array or not in user code.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines
Regarding doc: If this PR is accepted #21296 should be updated accordingly - or doc needs to be adapted in this PR if #21296 landed meanwhile.