bug: no way to supply default values for options set as multiple

[d4rckh]

there's a bug in argparse which won't let me supply default values for an option that i set as multiple. here's a small poc which will error:

import argparse

var p = newParser:
  option("-H", "--header", default=some(@["Content-Type: application/json"]), help="specify headers", multiple=true)
try:
    let parsedArgs = p.parse(commandLineParams())

    echo parsedArgs.header
except UsageError as e:
  stderr.writeLine getCurrentExceptionMsg()
  quit(1)

Error: type mismatch: got <string, string, default: Option[seq[string]], help: string, multiple: bool>
but expected one of:
proc option(name1: string; name2 = ""; help = ""; default = none[string]();
            env = ""; multiple = false; choices: seq[string] = @[];
            required = false; hidden = false)
  first type mismatch at position: 3
  required type for default: Option[system.string]
  but expression 'default = some(@["Content-Type: application/json"])' is of type: Option[seq[string]]
proc option[T](val: sink T): Option[T]
  first type mismatch at position: 2
  extra argument given

expression: option("-H", "--header", default = some(@["Content-Type: application/json"]),

If I change default to default=some("Content-Type: application/json"), the error will change:

Error: type mismatch: got 'string' for '"Content-Type: application/json"' but expected 'seq[string]'

Everything works as expected if I don't supply default