Only seal arrays and objects when a parser is immutable
Only seal arrays and objects when a parser is immutable

We seal nodes conditionally on the :immutable property, but we are
sealing their contents unconditionally.

https://bugzilla.gnome.org/show_bug.cgi?id=779970

frida/json-glib,ebassi/json-glib,ebassi/json-glib,frida/json-glib,GNOME/json-glib,GNOME/json-glib,ebassi/json-glib