Error with tuple as a dictionary key

[odow]

See JuliaCI/BenchmarkTools.jl#400 (comment)

(json) pkg> st
Status `/private/tmp/json/Project.toml`
  [682c06a0] JSON v1.1.0

julia> import JSON

julia> JSON.json(Dict(("a", "b") => 1))
ERROR: ArgumentError: No key representation for Tuple{String, String}. Define StructUtils.lowerkey(::Tuple{String, String})
Stacktrace:
 [1] lowerkey(::JSON.JSONWriteStyle, x::Tuple{String, String})
   @ JSON ~/.julia/packages/JSON/jLnej/src/write.jl:212
 [2] applyeach
   @ ~/.julia/packages/StructUtils/CvpPP/src/StructUtils.jl:563 [inlined]
 [3] json!(buf::Vector{…}, pos::Int64, x::Dict{…}, opts::JSON.WriteOptions{…}, ancestor_stack::Vector{…}, io::Nothing, ind::Int64, depth::Int64, bufsize::Int64)
   @ JSON ~/.julia/packages/JSON/jLnej/src/write.jl:617
 [4] json! (repeats 3 times)
   @ ~/.julia/packages/JSON/jLnej/src/write.jl:562 [inlined]
 [5] json(x::Dict{Tuple{String, String}, Int64}; pretty::Bool, kw::@Kwargs{})
   @ JSON ~/.julia/packages/JSON/jLnej/src/write.jl:433
 [6] json(x::Dict{Tuple{String, String}, Int64})
   @ JSON ~/.julia/packages/JSON/jLnej/src/write.jl:426
 [7] top-level scope
   @ REPL[5]:1
Some type information was truncated. Use `show(err)` to see complete types.

where previously

julia> import JSON

julia> JSON.json(Dict(("a", "b") => 1))
"{\"(\\\"a\\\", \\\"b\\\")\":1}"

[quinnj]

Fix proposed: #400