Set as Union value quirks

[alexdlm]

What happened?

Union values that are sets have a few quirks. eg:

MyUnion:
  union:
    fooSet: set<Foo>

They are not implemented as LinkedHashSet, and so do not retain insertion order like conjure'd set<> fields do.
They are not immutable copies (!).
They are slightly more difficult to construct (eg MyUnion.fooSet(...) the arg must be a set).

The generated FooSetWrapper:

    @JsonTypeName("fooSet")
    private static final class FooSetWrapper implements Base {
        private final Set<Foo> value;

        @JsonCreator(mode = JsonCreator.Mode.PROPERTIES)
        private FooSetWrapper(@JsonSetter(value = "fooSet", nulls = Nulls.AS_EMPTY) @Nonnull Set<Foo> value) {
            Preconditions.checkNotNull(value, "fooSet cannot be null");
            this.value = value;
        }
        // ...

Regular conjure objects are deserialized via their builder, which initializes sets as LinkedHashSet, copies the incoming value, and then on build stores an immutable set.

I would guess this behavior extends to Maps as union members also.

What did you want to happen?

1. Maintain insertion order like conjure fields. Probably this arg needs a @JsonDeserialize(as = LinkedHashSet.class).
2. Do not store this.value directly, make a copy.
3. Have additional Union creators that take Iterable like regular fields.