'x-forwarded-host' should be added to proxyHeadersIgnore by default

[hassanselim0]

The X-Forwarded-Host header is a very common header used by Reverse Proxies like Nginx and Cloudflare.
If I have both Nuxt and my API behind a reverse proxy, Nuxt gets a header like X-Forwarded-Host: my-nuxt-app.example.com so when making an API call this axios module happily copies that header over to the API Request.
When Nginx gets X-Forwarded-Host in a request it causes a lot of confusion which leads to it rewriting the Host header to be like Host: my-nuxt-app.example.com even though the request gets routed to the API Server.
This in turn confuses a lot of API Servers because they get an unexpected Host header value. Django for example has an ALLOWED_HOSTS config and uses it to secure against fake host headers.

So to summarize, I don't think there will ever be a case were a developer would want the X-Forwarded-Host header proxied by Axios, it's logically incorrect and causes a lot of confusion. This is why I think it should be part of the proxyHeadersIgnore by default and save other people the 5 hours I spent trying to debug the issue I had.

[atinux]

I think it’s good for me, PR welcome!

CC @pi0

[hassanselim0]

@pi0 You previously added a note in the docs for proxyHeaders saying to disable it when using CloudFlare (here e9e225f), but I think now that we have the cf-* headers and soon the x-forwarded-host header ignored by default then this note is not necessary anymore, right?
Maybe it can be replaced by another note explaining the importance of having these 3 headers ignored by default so people don't accidentally fall into this issue when replacing the array with their own one.
I haven't tested this under cloudflare yet so I'm not 100% sure about it though.

[hassanselim0]

@atinux @pi0 Here is the PR: #462