Serverspec::Filter.filter_subject supports other than int

Now you can check kernel parameters like this.

```ruby
describe 'Linux kernel parameters' do
  context 'kernel.osrelease' do
    its(:value) { should eq '2.6.32-131.0.15.el6.x86_64' }
  end

  context 'net.ipv4.tcp_wmem' do
    its(:value) { should match /4096\t16384\t4194304/ }
  end
end
```