Beware that unlike soup_message_headers_get(), this processes the
headers in exactly the way they were added, rather than
concatenating multiple same-named headers into a single value.
(This is intentional; it ensures that if you call
soup_message_headers_append() multiple times with the same name,
then the I/O code will output multiple copies of the header when
sending the message to the remote implementation, which may be
required for interoperability in some cases.)
Calls @func once for each header value in @hdrs.
Beware that unlike soup_message_headers_get(), this processes the headers in exactly the way they were added, rather than concatenating multiple same-named headers into a single value. (This is intentional; it ensures that if you call soup_message_headers_append() multiple times with the same name, then the I/O code will output multiple copies of the header when sending the message to the remote implementation, which may be required for interoperability in some cases.)
You may not modify the headers from @func.