SetMultimap should be deserialized to a LinkedHashMultimap by default

[twbecker]

Currently, when deserializing to a field declared simply as a SetMultimap, a HashMultimap is used as the implementation. This is at odds with Jackson's default behavior for normal Maps where a LinkedHashMap is used to preserve the ordering present in the JSON. It seems like the Guava module should analogously use LinkedHashMultimap as the default SetMultimap implementation.

[cowtowncoder]

Sounds reasonable to me; somehow did not notice this so apologies for ultra-slow follow up.
Seems like a simple change if anyone wants to have a look: should go in 2.12 branch to be included in 2.12.0 since it is behavior change (small but visible, likely to cause some false test fails somewhere, wrt fragile tests & ordering assumptions).

[Migwel]

I'd like to work on this, if it hasn't been picked up yet

[cowtowncoder]

@Migwel yes, go for it! :)