Dio is forcing headers to lowercase

Dio version: dio: ^3.0.8

When I set Dio header like this

dio.options.headers['Authorization'] = 'dGVzdDp0ZXN0';

I expect request header to look like this

Authorization: Basic dGVzdDp0ZXN0

Not like this (notice that capital "A" was transformed to "a")

authorization: Basic dGVzdDp0ZXN0

Reason for this is HERE. All headers are transformed to lower case.
I can only guess what your motivation for this was, but this is wrong.

Please read about it HERE and HERE

Dio 2.1.5 did not have this behavior. I was able to use AltHttpClient with Dio and headers where left AS IS.

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