`to_i` was added for SQLite3 adapter which did not handle number
but sqlite3 gem already supports it then `to_i` is unnecessary.

else condition is kept for adapters which return string,
i.e. mysql(not mysql2) and sqlserver.

Renamed `test_cache_does_not_wrap_string_results_in_arrays`
to `test_cache_does_not_wrap_results_in_arrays` to explain the
current behavior. most of adapters return integer, not only string.

* Refer these commits:

"future proofing the sqlite3 adapter code"
https://github.com/rails/rails/commit/beda2d43d6ac5c3435fc2fba0cbd108c20fe1c67

"Refactor calculation test to remove unneeded SQLite special case."
https://github.com/rails/rails/commit/47d568ed3fc701934ebe80b276f3d8bf6951c93f

"no need to to_i, sqlite does that for us"
https://github.com/rails/rails/commit/6cf44a1bd64ba10497742d70ad78fe68faa16e99