Fix thread spawning edge case.

Rapidly adding work to the ThreadPool can result in many jobs making it onto `@todo` before one of the jobs gets the mutex lock to decrement `@waiting`. So `@waiting == 0` isn't true and no thread is spawned even though there's work piling up, e.g.:

```ruby
require 'puma/thread_pool'

pool = Puma::ThreadPool.new(1, 3) { sleep 2 }

3.times { pool << 1 }

sleep 1

pool.spawned #=> 1 # When 3 is expected
```

Checking if `@waiting < @todo.size` shows that there's more work to do than threads waiting even if `@waiting` hasn't been decremented to `0` and also covers the base case where `@waiting == 0` and `@tudo.size == 1`.

An alternate option would be just adding the new check without removing the old one, something like `(@waiting == 0 or @waiting < @todo.size)`, but I don't think it's necessary unless for some kind of performance reason.
Merge pull request #570 from havenwood/spawn-thread-edge-case

Fix thread spawning edge case.