This is not we're supposed to do according to https://tc39.es/ecma262/#sec-runtime-semantics-propertydefinitionevaluation
Furthermore, this was observable by ToPrimitive looking up toString and
valueOf and potentially calling them if they exist. The big ticket
issue however is that for objects without toString and valueOf, such as
null-proto objects, this would unexpectedly throw.
Before this was a mix of different strategies but copy_data_properties
does all of that in a spec way.
This fixes numeric properties in object spreading. And ensures that any
new properties added during spreading are not taken into account.