CHEF-4927 - coerce group GID to string

To remain consistent with the way the user provider works, coerce both
new_resource and current_resource GIDs to strings for comparison. This
allows users to write their resources like:

```ruby
group "zone" do
  gid 233
end
```

or

```ruby
group "zone" do
  gid "233"
end
```

Without this change, every time Chef runs when a string GID is
specified, it will modify the group, which is not desirable.