Loop multiplicity incorrect

[scheinerman]

When adding a single loop to a Multigraph, its multiplicity ought to be one, not two:

julia> using Graphs, Multigraphs

julia> g = Multigraph(4)
{4, 0} undirected Int64 multigraph

julia> add_edge!(g,1,1)
true

julia> collect(edges(g))
1-element Vector{Any}:
 Multiple edge 1 => 1 with multiplicity 2