accept non-float z

[tpapp]

contours does not accept a z of Ints. A use case is count data:

using Contour
using StatsBase

N = 10000
x = randn(N)
y = randn(N)
H = fit(Histogram, (x, y), closed = :left)
contours(midpoints.(H.edges)..., H.weights)           # fails
contours(midpoints.(H.edges)..., Float64.(H.weights)) # works

The error message for the failing one is

julia> contours(midpoints.(H.edges)..., H.weights)
ERROR: MethodError: no method matching interpolate(::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}, ::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}, ::Array{Int64,2}, ::Float64, ::Int64, ::Int64, ::UInt8)
Closest candidates are:
  interpolate(::Any, ::Any, ::Array{T<:AbstractFloat,2}, ::Number, ::Int64, ::Int64, ::UInt8) where T<:AbstractFloat at /home/tamas/.julia/v0.6/Contour/src/Contour.jl:232
Stacktrace:
 [1] trace_contour(::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}, ::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}, ::Array{Int64,2}, ::Float64, ::Dict{Tuple{Int64,Int64},Contour.Cell}) at /home/tamas/.julia/v0.6/Contour/src/Contour.jl:328
 [2] collect(::Base.Generator{StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}},Contour.##1#2{StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}},StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}},Array{Int64,2}}}) at ./array.jl:475
 [3] contours at /home/tamas/.julia/v0.6/Contour/src/Contour.jl:81 [inlined]
 [4] contours at /home/tamas/.julia/v0.6/Contour/src/Contour.jl:88 [inlined]
 [5] contours(::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}, ::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}, ::Array{Int64,2}) at /home/tamas/.julia/v0.6/Contour/src/Contour.jl:94