Make show for Options work on Julia 1.0

`dateformat` can be `nothing` and in this case Julia 1.0 errors. This PR fixes the error.

src/CSV.jl

@@ -87,7 +87,7 @@ function Base.show(io::IO,op::Options)
     println(io, "        quotechar: '", Char(op.quotechar), "'")
     print(io, "        escapechar: '"); escape_string(io, string(Char(op.escapechar)), "\\"); println(io, "'")
     print(io, "        missingstring: \""); escape_string(io, isempty(op.missingstring) ? "" : String(collect(op.missingstring)), "\\"); println(io, "\"")
-    println(io, "        dateformat: ", op.dateformat)
+    println(io, "        dateformat: ", op.dateformat == nothing ? "" : op.dateformat)
     println(io, "        decimal: '", Char(op.decimal), "'")
     println(io, "        truestring: '$(String(op.truestring))'")
     println(io, "        falsestring: '$(String(op.falsestring))'")