Removes double occurence of features in feature list when string is used.

before
```ruby
@rollout.activate(:chat)
@rollout.activate("chat")

@rollout.features #=> [:chat, :chat]
```

after
```ruby
@rollout.activate(:chat)
@rollout.activate("chat")

@rollout.features #=> [:chat]
```

the first one will get merged anyway when call #activate again

```ruby
@rollout.activate(:chat)
@rollout.activate("chat")

@rollout.features #=> [:chat, :chat]

@rollout.activate(:chat)
@rollout.features #=> [:chat]
```
```