Printf bug for BigInt

[simeonschaub]

The following fails on master, as well as 1.4:

julia> using Printf

julia> @printf "%.5e" big(10)
ERROR: MethodError: no method matching decode_dec(::BigInt)
Closest candidates are:
  decode_dec(::BigInt, ::Any) at /home/simeon/Documents/Julia/julia/usr/share/julia/stdlib/v1.6/Printf/src/Printf.jl:932
  decode_dec(::Integer, ::Any) at /home/simeon/Documents/Julia/julia/usr/share/julia/stdlib/v1.6/Printf/src/Printf.jl:892
  decode_dec(::Real, ::Any) at /home/simeon/Documents/Julia/julia/usr/share/julia/stdlib/v1.6/Printf/src/Printf.jl:855
  ...
Stacktrace:
 [1] ini_dec(::BigInt, ::Int64, ::Array{UInt8,1}) at /home/simeon/Documents/Julia/julia/usr/share/julia/stdlib/v1.6/Printf/src/Printf.jl:1062
 [2] ini_dec(::Base.TTY, ::BigInt, ::Int64, ::String, ::Int64, ::Int64, ::Char, ::Array{UInt8,1}) at /home/simeon/Documents/Julia/julia/usr/share/julia/stdlib/v1.6/Printf/src/Printf.jl:845
 [3] top-level scope at REPL[6]:1

It seems like decode_dec(::BigInt) calls a method that does not exist.