according to documentation here:
https://doc.ez.no/eZ-Publish/Technical-manual/4.x/Reference/Configuration-files/site.ini/HTTPHeaderSettings/HeaderList

a custom header can be defined using syntax:

<HTTP header>[<eZ Publish path|module{/view}>]=<value>{;<depth>{;<level>}}

but, using this syntax declaring an header such as:
Content-Type:text/html; charset=iso-2022-jp

is not possible. the semicolon( ; ) is part of the header itself, but acts as separator for the HeaderList setting, so the resulting header is always just

 Content-Type:text/html; 

test case:
set site.ini -> HTTPHeaderSettings to

[HTTPHeaderSettings]
CustomHeader=enabled

HeaderList[]=Content-Type
Content-Type[]
Content-Type[/]=text/html; charset=UTF-16 #or other charset you like except uft-8

result: content-type will not show the declared charset