allow activate defined `let` by `let!` without block

In last few months I have a lot specs of next structure:

```ruby
describe 'Some specs' do
  let(:car) { create :car }

  it "spec which doesn't need `car`" do

  end

  it "spec which need `car` not instantly" do
    # some code
    car.turn_left
    # some code
  end

  context 'context which needs preloaded car' do
    let!(:car) { create :car }

  end

  context 'another context which needs preloaded car' do
    let!(:car) { create :car }

  end
end
```
Above example shows us the duplication. Another solution for this problem might be next

```ruby
describe 'Some specs' do
  let(:car) { create :car }

  it "spec which doesn't need `car`" do

  end

  it "spec which need `car` not instantly" do
    # some code
    car.turn_left
    # some code
  end

  context 'context which needs preloaded car' do
    before { car }

  end

  context 'another context which needs preloaded car' do
    before { car }

  end
end
```
But using before make specs little bit unreadable.

My solution is using `let!` without block

```ruby
describe 'Some specs' do
  let(:car) { create :car }

  it "spec which doesn't need `car`" do

  end

  it "spec which need `car` not instantly" do
    # some code
    car.turn_left
    # some code
  end

  context 'context which needs preloaded car' do
    let!(:car)

  end

  context 'another context which needs preloaded car' do
    let!(:car)

  end
end
```
It activates defined `let` with lefting specs clean.