Incorrect IP addreses in x-forwarded-for header for IPv6 addresses

- Function App version: 2.0
- Function App language: Node.js

I am relying on `x-forwarded-for` header to determine IP address of an HTTP request, and it works very well for IPv4 addresses. However, when I make a request from a device with an IPv6 address, the information in the header is wrong. Specifically, instead of containing my IPv6 address, it contains some other IPv4 address.

Start by reproducing an HTTP request to a Function App 2.0 endpoint from an IPv6 client and inspect the x-forwarded-for header, comparing it with an IPv4 request. Trace the Azure Functions host request handling that populates the header; done means the header preserves the originating IPv6 address instead of substituting an unrelated IPv4 address.

Written by the indexing model from the issue text.