gobject: Do not serialize default values
gobject: Do not serialize default values

If a property is set to its default value then we can skip its
serialization, to keep the number of JSON object members down.

brauliobo/json-glib,oerdnj/json-glib,Distrotech/json-glib,Distrotech/json-glib,GNOME/json-glib,ebassi/json-glib,oerdnj/json-glib,Distrotech/json-glib,oerdnj/json-glib,brauliobo/json-glib,ebassi/json-glib,oerdnj/json-glib,brauliobo/json-glib,frida/json-glib,GNOME/json-glib,frida/json-glib,ebassi/json-glib