Make sure HeaderMap adaptive hashing works.

[carllerche]

In theory it should, but it hasn't been tested. There should be unit tests to verify it works correctly.

[seanmonstar]

I believe the steps here are to find a bunch of header names that collide with the FNV hasher, insert them past the load limit, and then assert the danger field is Red with a SIP hasher (and all the keys have been re-hased).