`@quoted_{column,table}_names` should cache a frozen string

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`!!!!!!"
```

1 files changed, 2 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/mysql/quoting.rb b/activerecord/lib/active_record/connection_adapters/mysql/quoting.rb
index 5b59e39d9f..af1db30047 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/quoting.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/quoting.rb
@@ -5,11 +5,11 @@ module ActiveRecord
         QUOTED_TRUE, QUOTED_FALSE = '1'.freeze, '0'.freeze
 
         def quote_column_name(name)
-          @quoted_column_names[name] ||= "`#{super.gsub('`', '``')}`"
+          @quoted_column_names[name] ||= "`#{super.gsub('`', '``')}`".freeze
         end
 
         def quote_table_name(name)
-          @quoted_table_names[name] ||= super.gsub('.', '`.`')
+          @quoted_table_names[name] ||= super.gsub('.', '`.`').freeze
         end
 
         def quoted_true