Fixed parsing of incoming comma-separated list of Accept headers

[bart-degreed]

Our integration tests did actually test for compound values. But they use an internal loopback, which bypasses the actual Accept header parsing that occurs on incoming socket bytes.

Because httpContext.Request.Headers["Accept"] returns StringValues, I blindly assumed that this would break up comma-separated values. But it does not, as explained here. It only contains multiple entries when multiple Accept header lines are sent sequentially (or when short-circuited, as in our tests).

There seems no way to bypass the internal comma-separated values parsing that occurs in integration tests. Even passing a single comma-delimited string runs through an internal parser, producing a StringValues with multiple entries.

Fixes #970.