feat: recognise system proxy env var;

[LinHeLurking]

A simple commit that automatically selects proxy by environment variables.

[algesten]

Hi! Welcome to ureq!

We don't want to do this in our standard new() constructor. It introduces potentially unclear behavior where proxy settings are picked up without the user's control. I.e. it should be possible to create an AgentBuilder that ignores the environment variables.

I think this function should be broken into two halves.

1. Proxy::new_from_env() -> Result<Proxy>, i.e. move the env logic to a new constructor in Proxy.
2. A feature flag proxy-from-env that is on by default, and only affects the default agent (i.e. for ureq::get).

That way someone who is using a proxy, can bypass it either by deliberately clearing the feature flag, or by using their own AgentBuilder without setting the Proxy.

[LinHeLurking]

Hi! Welcome to ureq!

We don't want to do this in our standard new() constructor. It introduces potentially unclear behavior where proxy settings are picked up without the user's control. I.e. it should be possible to create an AgentBuilder that ignores the environment variables.

I think this function should be broken into two halves.

1. Proxy::new_from_env() -> Result<Proxy>, i.e. move the env logic to a new constructor in Proxy.
2. A feature flag proxy-from-env that is on by default, and only affects the default agent (i.e. for ureq::get).

That way someone who is using a proxy, can bypass it either by deliberately clearing the feature flag, or by using their own AgentBuilder without setting the Proxy.

Good suggestion! I've made the changes in new commit.

[algesten]

Thanks!

This was sorted in a slightly different way in #649