Multiline headers are allowed when at least one SP or HT character precedes additional lines of a header, as in the following example from the RFC:

Content-Type: Text/x-Okie; charset=iso-8859-1;
     declaration="<[email protected]>"

With this header, the s_header_field callback is invoked only once with the first line. Expected result would be to invoke it twice, once for each line, as happens with multiline part data.