MutableBinaryHeap doesn't work with Abstract / Union Type

[depial]

julia> h = MutableBinaryHeap{Union{Tuple{Int, Int}, Tuple{Float64,Float64}}}(Base.By(last), [(1,2),(1,1)])
MutableBinaryHeap((1, 1), (1, 2))

julia> push!(h, (1.5, 2.5))
MethodError: Cannot `convert` an object of type 
  DataStructures.MutableBinaryHeapNode{Tuple{Float64, Float64}} to an object of type 
  DataStructures.MutableBinaryHeapNode{Union{Tuple{Float64, Float64}, Tuple{Int64, Int64}}}
The function `convert` exists, but no method is defined for this combination of argument types.
[...]


julia> h2 = BinaryHeap{Union{Tuple{Int, Int}, Tuple{Float64,Float64}}}(Base.By(last), [(1,2),(1,1)])
BinaryHeap{Union{Tuple{Float64, Float64}, Tuple{Int64, Int64}}, Base.Order.By{typeof(last), Base.Order.ForwardOrdering}}(Base.Order.By{typeof(last), Base.Order.ForwardOrdering}(last, Base.Order.ForwardOrdering()), Union{Tuple{Float64, Float64}, Tuple{Int64, Int64}}[(1, 1), (1, 2)])

julia> push!(h2, (1.5, 2.5))
BinaryHeap{Union{Tuple{Float64, Float64}, Tuple{Int64, Int64}}, Base.Order.By{typeof(last), Base.Order.ForwardOrdering}}(Base.Order.By{typeof(last), Base.Order.ForwardOrdering}(last, Base.Order.ForwardOrdering()), Union{Tuple{Float64, Float64}, Tuple{Int64, Int64}}[(1, 1), (1, 2), (1.5, 2.5)])

There is a simple fix here.

[oxinabox]

closed in #869