cascalog.api/union (and combine) don't work with taps and flows

[ipostelnik]

cascalog.api/union and cascalog.api/combine only work with queries. A common pattern in our code is to do (combine (select-fields tap1 FIELDS) (select-fields tap2 FIELDS)). This construct used to work in 1.x, but doesn't in 2+. The workarounds are to drop down to cascading DSL or to wrap select with a noop query.