Maintain key order when restoring references

[eemeli]

Currently, when called with { references: true }, objects containing a repeated or circular reference will not have the property in question set until after the initialiser is done. This means that the key order may change from the original.

Now admittedly, the key order is not guaranteed by ECMAScript in the first place, but the fix that's required is really rather minimal; just put in an undefined value during the initialiser and replace that in the subsequent assignment. This would also make objects more closely match the current behaviour of arrays.

[coveralls]

Coverage remained the same at 98.755% when pulling d8c8305 on eemeli:maintain-order into 3155073 on blakeembrey:master.

[blakeembrey]

Thanks @eemeli!