Caching a mutable string causes the following issue.

```
Loading development environment (Rails 5.1.0.alpha)
irb(main):001:0> ActiveRecord::Base.connection.quote_table_name('foo') << '!!'
=> "`foo`!!"
irb(main):002:0> ActiveRecord::Base.connection.quote_table_name('foo') << '!!'
=> "`foo`!!!!"
irb(main):003:0> ActiveRecord::Base.connection.quote_table_name('foo') << '!!'
=> "`foo`!!!!!!"
```