Fix Accept header sort order #361

[u5surf]

Fixes: #361

• Implements Clone for Accept in order to copy it to tmporary variable and evaluate with it inside negotiate method, so that self cannot be sorted each negotiate is called.

• Add a test which evaluates negotiation properly the header which has same weight.

[yoshuawuyts]

Thanks for opening this PR! On reviewing the spec, you're indeed right that our current implementation is not compliant - but this PR doesn't implement the ordering dictated by the spec either.

I've written an analysis of the issues we have in our content negotiation in https://github.com/http-rs/http-types/issues/361#issuecomment-1008872107, and my hope is that we can use that to implement a spec-compliant fix.

I'm going to close this PR for now since it doesn't match the spec. But I'd be keen to review any follow-up PRs which help bring our content-negotiation implementation according to spec. Thanks!