add a test that there are no Core.Box in Downloads.jl (#289)

Author: KristofferC

test/setup.jl

@@ -8,6 +8,10 @@ using Base.Experimental: @sync
 
 include("json.jl")
 
+if isdefined(Test, :detect_closure_boxes)
+    @test isempty(Test.detect_closure_boxes(Downloads))
+end
+
 function download_body(url::AbstractString; kwargs...)
     sprint() do output
         download(url, output; kwargs...)