If given a single querier, just return it instead of constructing a
complicated wrapper. The code in `mergeGenericQuerier` which skipped
merging when there was only one is not needed any more.
This change required a few tests to be tweaked, because they relied on
the specific behaviour of `mergeGenericQuerier.Select()`.
Signed-off-by: Bryan Boreham <bjboreham@gmail.com>