nmcli: fix handling of connection.down-on-poweroff property
Summary
Fix the following error:
$ nmcli connection modify C connection.down-on-poweroff yes
**
nm:ERROR:../src/libnmc-setting/nm-meta-setting-desc.c:1367:_set_fcn_gobject_ternary: assertion failed: (_gobject_property_get_gtype(G_OBJECT(setting), property_info->property_name) ==
Bail out! nm:ERROR:../src/libnmc-setting/nm-meta-setting-desc.c:1367:_set_fcn_gobject_ternary: assertion failed: (_gobject_property_get_gtype(G_OBJECT(setting), property_info->property
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