fun ApplicationCall.defaultTextContentType(
    contentType: ContentType?
): ContentType

Creates a default ContentType based on the given contentType and current call

If contentType is null, it tries to fetch already set response header “Content-Type”. If the header is not available text/plain is used. If contentType is specified, it uses it

Additionally, if charset is not set for either content type, it appends ; charset=UTF-8 to the content type.