`new_constructor()` drops properties that have a getter, even if they have a setter

[lawremi]

See this line:

79557c0#diff-4b9b805aa09ee8da593ca9764b00b50a94ac6e0e1e4483ca63cfcb523acce9c9R58

My understanding is that a property is only read-only if it has a getter but not a setter. Perhaps it would be worth defining a prop_is_read_only() function?

[t-kalinowski]

Fixed in #445