Fix MutableBinaryHeap{T} when T is Union/abstract

Each MutableBinaryHeapNode needs to be fully parameterised or Julia will
throw a conversion error when trying to add them to MutableBinaryHeap's
nodes field.

src/heaps/mutable_binary_heap.jl

@@ -220,7 +220,7 @@ function Base.push!(h::MutableBinaryHeap{T}, v) where T
  nodemap = h.node_map
  i = length(nodemap) + 1
  nd_id = length(nodes) + 1
- push!(nodes, MutableBinaryHeapNode(convert(T, v), i))
+ push!(nodes, MutableBinaryHeapNode{T}(convert(T, v), i))
  push!(nodemap, nd_id)
  _heap_bubble_up!(h.ordering, nodes, nodemap, nd_id)
  return i
@@ -260,7 +260,7 @@ function update!(h::MutableBinaryHeap{T}, i::Int, v) where T
  nd_id = nodemap[i]
  v0 = nodes[nd_id].value
  x = convert(T, v)
- nodes[nd_id] = MutableBinaryHeapNode(x, i)
+ nodes[nd_id] = MutableBinaryHeapNode{T}(x, i)
  if Base.lt(ordering, x, v0)
  _heap_bubble_up!(ordering, nodes, nodemap, nd_id)
  else

test/test_mutable_binheap.jl

@@ -342,4 +342,14 @@ end
  update!(h, 2, 20)
  @test isequal(heap_values(h), [0.5, 10.1, 3.0, 20.0])
  end
+
+ @testset "T is a Union" begin
+ h = MutableBinaryMinHeap{Union{Int, Float64}}()
+ push!(h, 1)
+ push!(h, 2.0)
+ update!(h, 1, 1.5)
+ update!(h, 2, 3)
+ @test pop!(h) === 1.5
+ @test pop!(h) === 3
+ end
 end # @testset MutableBinheap