Select dataset arrays before merging via dataset[key] = value

It did not really make sense that adding a dataset array to another dataset
with `Dataset.__setitem__` by default merged all dataset variables. With this
commit, set item only merges `dataset_array.select().dataset` (e.g., the array
variable itself and any coordinates). If you really want to merge in the full
dataset associated with a dataset array, there is still `Dataset.merge`, but I
expect this behavior will be less surprising.