The spec currently declares:

Header values are assumed to be in a mixed format, meaning that a normally comma separated header may appear as a single entry in the values array, one entry per value, or a mixture of the two. (e.g. new string[1] {"value, value, value"}, new string[3] {"value", "value", "value"}, or new string[2] {"value, value", "value"})

However, as @kolektiv pointed out, that's not entirely clear. My best recollection is that OWIN would not change the header values, just pull all headers with the same header name into a single array, and the framework would then need to process the header values as received by the server.

I propose we amend to the following:

Header values are assumed to be in a mixed format, meaning that a normally comma separated header may appear as a single entry in the values array, one entry per value, or a mixture of the two (e.g. new string[1] {"value, value, value"}, new string[3] {"value", "value", "value"}, or new string[2] {"value, value", "value"}). Servers conforming to OWIN SHOULD NOT change the header values received in the HTTP request and SHOULD expose the header values as received.

I'm happy to change the above from SHOULD to MUST.