fix: setCharset function does not preserve case of original Content-Type

[albertoborit]

Adds or replaces the charset in a Content-Type string while preserving the original casing (e.g., "TEXT/PLAIN" stays "TEXT/PLAIN").
Fixes (if needed yet) #6613

cc: @wesleytodd @UlisesGascon

[MohamedM216]

Since converting Content-Type headers to lowercase is considered a good practice (as mentioned here #6613 (comment)), I believe these changes are unnecessary.