Enforcing the Charset on Content-Type is breaking the request. #2129
Description
The forced addition of a charset parameter in updateCharset() introduces a regression for integrations where the server expects an exact Content-Type without additional parameters.
The intent of the change (ensuring a deterministic charset for text media types) is valid and RFC-aligned, but it breaks interoperability with systems that do strict string matching on the Content-Type header.
Since AHC is widely used for legacy and enterprise systems (e.g., SAP, telecom integrations, XML gateways), the client should not mutate the Content-Type if the user has explicitly set it.
Expected behavior:
If the user sets a Content-Type manually, AHC should not alter it.
Actual behavior:
AHC rewrites text/* content types by forcefully appending ; charset=UTF-8, even when the caller does not want it.