* correct reference for `having count(*)`
It turns out that `having count(*) ...` was always treating
the count(*) as exactly 1. After studying this a lot, I noticed
that in fact, we correctly handle other counts. The reason is
that there's already code to recognize aggregates in `having`
clauses as matching aggregates that are being computed -- but
it only covers the other aggregate clause types, not the newly
added `countStarPlanExpression` from making `count(*)` work even
if there's no `_id` field.
We add several corresponding test cases.
* fix sum(a_decimal) type conversion
Added a test case for this, and also added a fix for it.
Underlying issue: qualifiedRefPlanExpression could end up
producing an int64 instead of a pql.Decimal, even though it
had expected type Decimal.
Originally this worked by politely converting an int64 to
a pql.Decimal in the Evaluate phase, but this was not ideal;
the real question is why it was coming out as an int64 at
that step. Showed this to Pat, who spent a while studying it
and produced a better fix.
* temporarily comment out test which fails in DAX
