Update combine to use splat ... style vcat, like in Base.vcat

This change is helpful to support changing vcat to be more consistent
with Base.vcat, where passing an array is not allowed.

Author: cjprybol

src/groupeddatatable/grouping.jl

@@ -193,7 +193,7 @@ combine(map(d -> mean(dropnull(d[:c])), gd))
 """
 function combine(ga::GroupApplied)
     gd, vals = ga.gd, ga.vals
-    valscat = vcat(vals)
+    valscat = vcat(vals...)
     idx = Vector{Int}(size(valscat, 1))
     j = 0
     @inbounds for (start, val) in zip(gd.starts, vals)

test/grouping.jl

@@ -146,7 +146,7 @@ module TestGrouping
     groupby(dt, [:v1, :v2])
 
     dt2 = by(e->1, DataTable(x=Int64[]), :x)
-    @test size(dt2) == (0,1)
+    @test size(dt2) == (0,2)
     @test isequal(sum(dt2[:x]), Nullable(0))
 
     # Check that reordering levels does not confuse groupby