Dont use Enumarator on join_association

Fibers are evil .. AR should avoid them when possible.

Didnt full branchmark this, but just running the tests we can see a difference:
`time bundle exec rake test_sqlite3`
```
Without the patch:
real0m41.930s
user0m32.107s
sys0m2.815s
```
```
With the patch:
real0m41.203s
user0m31.371s
sys0m2.772s
```

review @rafaelfranca @tenderlove