bug printing iterable index sets

[mlubin]

julia> using JuMP

julia> m = Model()
Feasibility problem with:
 * 0 linear constraints
 * 0 variables
Solver is default solver

julia> d = Dict(:a => 1)
Dict{Symbol,Int64} with 1 entry:
  :a => 1

julia> @variable(m, x[keys(d)])
x[i] free ∀ i ∈ {…}

julia> @variable(m, y[1:2])
2-element Array{JuMP.Variable,1}:
Error showing value of type Array{JuMP.Variable,1}:
ERROR: MethodError: `getindex` has no method matching getindex(::Base.KeyIterator{Dict{Symbol,Int64}}, ::Int64)
 in fill_var_names at /home/mlubin/.julia/v0.4/JuMP/src/print.jl:314
 in var_str at /home/mlubin/.julia/v0.4/JuMP/src/print.jl:302
 in showcompact_lim at show.jl:120
 in sprint at iostream.jl:206
 in alignment at show.jl:1012
 in print_matrix at show.jl:1087
 in print_matrix at show.jl:1079
 in anonymous at replutil.jl:23
 in with_output_limit at ./show.jl:1276
 in writemime at replutil.jl:23
 in display at REPL.jl:114
 in display at REPL.jl:117
 [inlined code] from multimedia.jl:151
 in display at multimedia.jl:163
 in print_response at REPL.jl:134
 in print_response at REPL.jl:121
 in anonymous at REPL.jl:624
 in run_interface at ./LineEdit.jl:1610
 in run_frontend at ./REPL.jl:863
 in run_repl at ./REPL.jl:167
 in _start at ./client.jl:420