Describe the bug
I cannot use the option DNSDefaultRoute= in systemd.network.networks.<name>.networkConfig.

To Reproduce
Steps to reproduce the behavior:

1. Set systemd.network.networks.<name>.networkConfig = { DNSDefaultRoute = true; };
2. Notice the error message Systemd Network has extra fields [DNSDefaultRoute].

Expected behavior
We should be able to use the option DNSDefaultRoute=.

Additional context
The problem seems to originate from the following stale assertion:

Notify maintainers
@arianvp @flokli @andir

Metadata
Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

 - system: `"x86_64-linux"`
 - host os: `Linux 5.4.49, NixOS, 20.03post-git (Markhor)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.6`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:

# a list of nixos modules affected by the problem
module:
- nixos/networkd