Allow check_is_(grouped,owned_by) matchers to use numeric GID/UID

This change will allow to use numeric `uid` and `gid` with the file's matchers `be_owned_by` and `be_grouped_into`.

```ruby
describe file('/path/file') do
  it { should be_owned_by '1001' }
end
```

The `uid` or `gid` can be used as *String* or *Numeric*.

```ruby
describe file('/path/file') do
  it { should be_grouped_into 1002 }
end
```

This can be useful to test files or directories inside of Linux Container where the users doesn't have to correlate with the ones in the host.