Fix `union` precedence

The precedence of data overwriting in union of DataGraphs is inconsistent with union for other types:

julia> using DataGraphs

julia> g1 = DataGraph(4);

julia> g2 = DataGraph(4);

julia> g1[1 => 2] = 1;

julia> g2[1 => 2] = 2;

julia> union(g1, g2)
DataGraph{Int64, Any, Any, Graphs.SimpleGraphs.SimpleGraph{Int64}, Graphs.SimpleGraphs.SimpleEdge{Int64}} with 4 vertices:
Base.OneTo(4)

and 0 edge(s):

with vertex data:
0-element Dictionaries.Dictionary{Int64, Any}

and edge data:
1-element Dictionaries.Dictionary{Graphs.SimpleGraphs.SimpleEdge{Int64}, Any}
 Edge 1 => 2 │ 2

compared to:

julia> union(Any[1], Any[1.0])
1-element Vector{Any}:
 1

Start by reproducing the two union examples in the issue and locate the DataGraph union entry point. Compare its overwrite behavior with Julia's union example; done means the DataGraph result follows the same precedence as union for other types, with the behavior covered by a regression check.