Fix PriorityQueue(ks, vs) constructor

Right now, the constructor for building a PriorityQueue from a list of keys and a list of values is broken:
````
julia> pq = PriorityQueue(1:10, 1:10)
ERROR: no method PriorityQueue{Int64,Int64}(Zip2{UnitRange{Int64},UnitRange{Int64}})
 in PriorityQueue at collections.jl:132
 in PriorityQueue at collections.jl:159
````
This patch fixes it and adds a test.

By the way, there's a comment above this constructor that says "maybe deprecate." What's the reason? It seems useful.