KeyError: key Irrational{:π} not found when passing an Irrational as an attribute

[ali-ramadhan]

Describe the bug

It seems you cannot save an Irrational as an attribute.

To Reproduce

using NCDatasets
ds = NCDataset("/tmp/test.nc", "c")
defDim(ds, "x", 10)
v = defVar(ds, "food", Float32, ("x",), attrib = Dict("pie" => π))

Expected behavior

I guess the error makes sense since the NetCDF library doesn't know about Julia's Irrational type and C does not have an irrational type.

As a package nicety would it make sense to convert Irrationals to Float64 for this use case?

Environment

$ uname -a
Linux manjaro 5.0.15-1-MANJARO #1 SMP PREEMPT Fri May 10 19:51:04 UTC 2019 x86_64 GNU/Linux

               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.5.1 (2020-08-25)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

(@v1.5) pkg> st
Status `~/.julia/environments/v1.5/Project.toml`
  [fbb218c0] BSON v0.2.6
  [6e4b80f9] BenchmarkTools v0.5.0
  [aae7a2af] DiffEqFlux v1.23.0
  [e30172f5] Documenter v0.25.2
  [442a2c76] FastGaussQuadrature v0.4.2
  [f6369f11] ForwardDiff v0.10.12
  [98b081ad] Literate v2.7.0
  [85f8d34a] NCDatasets v0.10.4
  [d848d694] OceanTurb v0.3.3
  [9e8cae18] Oceananigans v0.40.0
  [5fb14364] OhMyREPL v0.5.8
  [91a5bcdd] Plots v1.6.3
  [c3e4b0f8] Pluto v0.11.14
  [f27b6e38] Polynomials v1.1.7
  [92933f4c] ProgressMeter v1.4.0
  [438e738f] PyCall v1.92.1
  [d330b81b] PyPlot v2.9.0
  [295af30f] Revise v2.7.6
  [24249f21] SymPy v1.0.28
  [a759f4b9] TimerOutputs v0.5.6
  [e88e6eb3] Zygote v0.5.8
  [d6f4376e] Markdown

Full output

ERROR: KeyError: key Irrational{:π} not found
Stacktrace:
 [1] getindex at ./dict.jl:467 [inlined]
 [2] nc_put_att(::Int32, ::Int32, ::String, ::Irrational{:π}) at /home/alir/.julia/packages/NCDatasets/HhdCu/src/netcdf_c.jl:524
 [3] setindex!(::NCDatasets.Attributes{NCDataset{Nothing}}, ::Irrational{:π}, ::String) at /home/alir/.julia/packages/NCDatasets/HhdCu/src/attributes.jl:57
 [4] defVar(::NCDataset{Nothing}, ::String, ::DataType, ::Tuple{String}; kwargs::Base.Iterators.Pairs{Symbol,Dict{String,Irrational{:π}},Tuple{Symbol},NamedTuple{(:attrib,),Tuple{Dict{String,Irrational{:π}}}}}) at /home/alir/.julia/packages/NCDatasets/HhdCu/src/cfvariable.jl:158
 [5] top-level scope at REPL[13]:1