define method when method missing

We use connection_pool in our server.

```ruby
$redis = ConnectionPool::Wrapper.new(size: 5, timeout: 3) { Redis.connect }
$redis.sadd('foo', 1)
```
In fact, the method named `sadd` is one of Redis.connect`s methods which is delegated by ConnectionPool::Wrappe#method_missing.

It's more efficient to define the method when method missing.