Sets the body of this.
As a side-effect the [enum@Gio.DBusMessageHeaderField.SIGNATURE] header field is set to the type string of body (or cleared
if body is `NULL`).
If body is floating, this assumes ownership of body.
| this |
a D-Bus message |
| body |
a [type@GLib.Variant] containing a tuple, or `NULL` if no body is needed |