OrderedDict output changes on 1.4.0 -> 1.5.0

[penelopeysm]

On JSON.jl 1.4.0 and 1.5.0 the serialisation of OrderedDict changes:

julia> d = OrderedDict(:b => 2, :a => 1)
OrderedDict{Symbol, Int64} with 2 entries:
  :b => 2
  :a => 1

julia> JSON.json(d) # 1.4.0
"{\"b\":2,\"a\":1}"

julia> JSON.json(d) # 1.5.0
"{\"a\":1,\"b\":2}"

That means that roundtrip serialisation/deserialisation is no longer faithful.

I can see that this is a result of #442 and I welcome that added API, but the problem is that the new default leads to different semantics for containers whose iteration order is known. While one could argue that the serialisation order was never really guaranteed, the existing semantics in JSON.jl <= 1.4 are that it follows the iteration order, and I'd be surprised if I was the only user who relied on this. From my perspective this is effectively a breaking change.

In fact, Claude's writeup on #442 says that

JSON.Object is exempted because it was specifically designed for insertion-order preservation — users who choose Object have explicitly opted into a particular key ordering.

But that probably isn't the only collection where users have "explicitly opted into a particular key ordering". I'm using OrderedDict right now, but there may well be more stuff out there.

Would there be a more conservative way of making the same change, e.g. retaining the old behaviour as the default, and forcing the user to specify sort_keys=True if they actually want it to be sorted?