Set proxy host header to the exact value of the request host header

[kate-osborn]

Proposed changes

Problem: The Gateway API expects the response host header to match the exact value of the request host header. We set the proxy host header to the $host nginx variable, which does not include any characters after :. This causes a conformance test to fail where the host header contains a port.

Solution: Set the proxy host header to $http_host nginx variable when it is non-empty. This variable contains the unmodified host header of the request. However, in an HTTP/1.0 request, it's possible that $http_host can be empty. In this case, we will use the value of $host. See http://nginx.org/en/docs/http/ngx_http_core_module.html#var_host.

Testing: Ran impacted conformance test

--- PASS: TestConformance (22.30s)
    --- PASS: TestConformance/HTTPRouteHostnameIntersection (9.04s)

Closes #797

Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
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