[Perf] Don't use Enumerable#next in pluck since it is very slow

Similar to #12090, using Enumerable#next in the pluck method creates many extra Fibers which drastically affects performance.

Here are some benchmarks

```
Rails master
3.times { Item.pluck(:id) } #=> 2.080483

Rails master with optimization (pull request)
3.times { Item.pluck(:id) } #=> 0.025729
```

Here is a gist to replicate my results.
https://gist.github.com/rywall/6395171