Prevent Symbol GC

Test failure in ruby-head , I think maybe that it caused Symbol GC.

https://travis-ci.org/rails/rails/jobs/42937185#L207-L210

```
  1) Failure:
FinderTest#test_symbols_table_ref [/home/travis/build/rails/rails/activerecord/test/cases/finder_test.rb:59]:
Expected: 33348
  Actual: 33345
```