Allowing compute to persist tables with Spark SQL backend

[zacdav-db]

Fix for #1502

Previously this example would fail, it now succeeds:

# I have results I don't want to collect but I do want to persist
results <- tbl(con, I("samples.nyctaxi.trips")) %>%
  group_by(pickup_zip) %>%
  summarise(avg_trip_dist = mean(trip_distance)) %>%
  compute(
    name = I("zacdav.default.avg_trip_dist"),
    temporary = FALSE,
    overwrite = TRUE
  )