unique()-ing records breaks es_search() as of data.table 1.12.0

[jameslamb]

Earlier today @fparages noted that uptasticsearch::es_search() was throwing an error like this:

Error in forderv(x, by = by, sort = FALSE, retGrp = TRUE) :
Column 2 of by= (2) is type 'list', not yet supported

We tracked this down to only occurring when the result had hits where at least one field was an array. In Felipe's case, this was occurring when he use sorting and every hit then came back with this in it:

"sort": [
    "all",
    "some_id"
]

After some investigation tonight, I discovered Rdatatable/data.table#3332. Basically running unique(someDT) when someDT contains any columns that are lists will throw an error as of data.table 1.12.0 (released two weeks ago).