Cleanup environment variables

[casconed]

Add support for already-documented variable GUZZLE_CURL_SELECT_TIMEOUT , and add additional documentation for NO_PROXY

[FrontEndCoffee]

I am personally not a fan of else and elseif statements, but I don't see how in this situation the readability could be improved without adding additional methods (and thus complication).

Looks good 👍

[sagikazarmark]

Thanks @casconed

[wirwolf]

Hello @casconed, i create a fork of this repo and release this marge request into the new version(6.4.0).

Namespace compatibility is 100%, and you can use my changes if you patch composer.json in your projects

• replace "guzzlehttp/guzzle": "^6.3" to "someblackmagic/guzzle": "^6.4"
• add "guzzlehttp/guzzle": "6.3.*" into replace block

Fork link: https://github.com/SomeBlackMagic/guzzle/