fix: multiple default values combined with ArgEnum / PossibleValues

[plaflamme]

Fixes a regression when combining ArgEnum and a default_value with multiple values, e.g.:

#[derive(ArgEnum)]
enum Arg {
  Foo,
  Bar,
}

#[derive(Parser)]
struct Opts {
  #[clarp(arg_enum, default_value="foo,bar", value_delimiter=',')]
  arg: Vec<Arg>
}

This results in the following error and backtrace:

thread 'arg_enum::vec_type_default_value' panicked at 'Argument `arg`'s default_value=foo,bar doesn't match possible values', src/build/debug_asserts.rs:691:21
stack backtrace:
   0: rust_begin_unwind
             at /rustc/8769f4ef2fe1efddd1f072485f97f568e7328f79/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/8769f4ef2fe1efddd1f072485f97f568e7328f79/library/core/src/panicking.rs:143:14
   2: clap::build::debug_asserts::assert_defaults
             at ./src/build/debug_asserts.rs:691:21
   3: clap::build::debug_asserts::assert_arg
             at ./src/build/debug_asserts.rs:623:5
   4: clap::build::debug_asserts::assert_app
             at ./src/build/debug_asserts.rs:57:9
   5: clap::build::command::App::_build
             at ./src/build/command.rs:4101:13
   6: clap::build::command::App::_do_parse
             at ./src/build/command.rs:3965:9
   7: clap::build::command::App::try_get_matches_from_mut
             at ./src/build/command.rs:681:9
   8: clap::build::command::App::try_get_matches_from
             at ./src/build/command.rs:597:9
   9: clap::derive::Parser::try_parse_from
             at ./src/derive.rs:126:23
  10: derive::arg_enum::vec_type_default_value
             at ./tests/derive/arg_enum.rs:480:9
  11: derive::arg_enum::vec_type_default_value::{{closure}}
             at ./tests/derive/arg_enum.rs:462:1
  12: core::ops::function::FnOnce::call_once
             at /rustc/8769f4ef2fe1efddd1f072485f97f568e7328f79/library/core/src/ops/function.rs:227:5
  13: core::ops::function::FnOnce::call_once
             at /rustc/8769f4ef2fe1efddd1f072485f97f568e7328f79/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

This is essentially the same issue as #3514, but with PossibleValues instead of FromStr.

[epage]

Thanks! I missed this case in #3521

[epage]

v3.1.6 is now released with this fix