Not entirely sure what the HTTP spec status of response headers is but with request headers

Key: Value1, Value2

is equivalent to:

Key: Value1
Key: Value2

Quite a few deprecation messages contain a comma which causes an ambiguity when I get to actually parse these (from .NET BCL in this case.

e.g:

HTTP/1.1 200 OK
Warning: Deprecated field [fielddata_fields] used, expected [docvalue_fields] instead
Warning: [groovy] scripts are deprecated, use [painless] scripts instead
content-type: application/json; charset=UTF-8
content-length: 3676

Yields:

Which is impossible to stitch back together as two messages.