if given, must be a property list of header keys and values
property key -- XMLRPC Request (new in 1.53)
a property list of other header elements (optional)
User-Agent: if not specified in the Header property, the SenseTalk long version will be supplied, e.g.: SenseTalk Release 1.53
property key in post multipart content (new in 1.54)
A property list defining additional header fields. This is only required if headers with the same name as other keys in the content property list are needed. Otherwise, header key/value pairs can be included directly in the content property list.