Prevent write queries with prepared statements for mysql2 adapter

Without this change, mysql2 adapter with prepared statements won't pass
`base_test.rb`.

```
% ARCONN=mysql2 be ruby -w -Itest test/cases/base_test.rb
Using mysql2
Run options: --seed 27614

# Running:

....S..............................F

Failure:
BasicsTest#test_creating_a_record_raises_if_preventing_writes [test/cases/base_test.rb:1493]:
ActiveRecord::ReadOnlyError expected but nothing was raised.

rails test test/cases/base_test.rb:1492

...F

Failure:
BasicsTest#test_deleting_a_record_raises_if_preventing_writes [test/cases/base_test.rb:1513]:
ActiveRecord::ReadOnlyError expected but nothing was raised.

rails test test/cases/base_test.rb:1510

............................................................................................................F

Failure:
BasicsTest#test_updating_a_record_raises_if_preventing_writes [test/cases/base_test.rb:1503]:
ActiveRecord::ReadOnlyError expected but nothing was raised.

rails test test/cases/base_test.rb:1500

..........

Finished in 2.534490s, 62.7345 runs/s, 149.5370 assertions/s.
159 runs, 379 assertions, 3 failures, 0 errors, 1 skips
```

0 files changed, 0 insertions, 0 deletions