Relation#respond_to? should bubble to parent

When using a Her::Model::Relation instance with RSpec expectations, this sort of test will fail:

```ruby
describe MyObject do
  describe '.all' do
    it 'should return all my fixtures' do
      expect(described_class.all).to have(5).items
    end
  end
end
```

It'll fail because the Her::Model::Relation instance looks like it doesn't implement `#size` or `#length`, so RSpec will try to call the `items` method on it. In turn, Her::Model::Relation will send the `items` method on to the Her::Collection. You'll get this (fairly opaque) error back:

```
Failure/Error: expect(described_class.all).to have(5).items
NoMethodError:
  undefined method `items' for #<Her::Collection:0x007fd5e22a5e80>
```

This patch adds a regression test and fixes it.