getHeaders
Obtains the headers for the part currently being processed.
Note that the struct@MessageHeaders that are returned are owned by the #SoupMultipartInputStream and will be replaced when a call is made to method@MultipartInputStream.next_part or its async counterpart, so if keeping the headers is required, a copy must be made.
Note that if a part had no headers at all an empty struct@MessageHeaders will be returned.
Return
a #SoupMessageHeaders containing the headers for the part currently being processed or null if the headers failed to parse.