hash policy should look at all values of a header

[markdroth]

The current hash policy code only looks at the first value of a given header. The code even has a TODO about this:

envoy/source/common/http/hash_policy.cc

Line 42 in 76286f6

// TODO(mattklein123): Potentially hash on all headers.

For consistency with the changes that were recently made in routing to always look at multiple values of a given header, I think we should do the same thing here.

CC @htuch

[htuch]

Yeah, this seems a bug to me, we should hash on the full header as per @mattklein123