If you don't absolutely need it, DON'T USE THIS PACKAGE.
This package is simply snapshot of the part of default Dart HTTP Client taken from the current master with patch TO ALLOW CASE SENSITIVE REQUEST HEADERS.
Default client forces all headers to lowercase which can create problems with some servers (IE. you can't perform basic authorization because server expects 'Authorization' header, not 'authorization'. You can read more about it HERE and HERE. If you possible can (not everyone can), try to contact server owner to fix server behavior.
If everything else fails, and you have no other options you can use this package as a simple replacement for default HTTP client. It inherits from abstract HTTP client just as default one, just instead of HttpClient() from dart:io package call AltHttpClient(). It's been tested and working with Dio. You can find example HERE.
Since it's taken from current master snapshot it requires Dart 2.8 due to breaking changes in HttpClient package. This package should add about 250 kb to your app when compiled. I've copied only neccessary parts, everything else is called from default dart:io namespace (IE. websockets).
I have no intention of updating and maintaining this package unless I personally need it.