Removed .toJSON() usage in wikibase.SnakList.getPropertyOrder

(bug 56673)
Not calling .toJSON() prevents serializing each snak which triggers an Error if
the snak features an UnUnserialzableValue.

Change-Id: Ied888ac7e1962332b6915a36238094a6879b71de
Merge "Removed .toJSON() usage in wikibase.SnakList.getPropertyOrder"