Keyword `decimal` not respected for AbstractFloats in CSV.write()

[sara2512]

Currently DecFP.Dec64 Floats are not handled correctly. Decimal character is not replaced.

using DecFP
using DataFrames
using CSV

io = IOBuffer() 
CSV.write(io, DataFrame(:a => Dec64(1)); decimal = ',')
String(take!(io))
# "a\n1.0\n"