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
```

activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb

@@ -122,6 +122,10 @@ def with_multi_statements
           end
 
           def exec_stmt_and_free(sql, name, binds, cache_stmt: false)
+            if preventing_writes? && write_query?(sql)
+              raise ActiveRecord::ReadOnlyError, "Write query attempted while in readonly mode: #{sql}"
+            end
+
             # make sure we carry over any changes to ActiveRecord::Base.default_timezone that have been
             # made since we established the connection
             @connection.query_options[:database_timezone] = ActiveRecord::Base.default_timezone