During the response process (especially setBody), HTTPResponse accesses
and updates some response headers in its "headers" property (a dictionary).
addHeader puts the response headers in a list which will not be updated by
HTTPResponse. This is "more correct" from an RFC perspective, as any header
specified as being a sequence of values delimited by commas may be split
among multiple headers.
So, keep using addHeader by default, but special-case some headers which
are accessed and must be successfully updated by HTTPResponse itself so
that those headers are set using setHeader, which updates the "headers"
property.