// This method changes options of the download denoted by gid (string) dynamically. options is a struct.
// The following options are available for active downloads:
//
// bt-max-peers
// bt-request-peer-speed-limit
// bt-remove-unselected-file
// force-save
// max-download-limit
// max-upload-limit
//
// For waiting or paused downloads, in addition to the above options, options listed in Input File subsection are available, except for following options: dry-run, metalink-base-uri, parameterized-uri, pause, piece-length and rpc-save-upload-metadata option.
// In addition, options listed in the Input File subsection are available, except for following options: checksum, index-out, out, pause and select-file.
// With the log option, you can dynamically start logging or change log file.
// To stop logging, specify an empty string("") as the parameter value.