Quoting booleans should return a frozen string

If reuse `QUOTED_TRUE` and `QUOTED_FALSE` without frozen, causing the
following issue.

```
Loading development environment (Rails 5.1.0.alpha)
irb(main):001:0> ActiveRecord::Base.connection.quote(true) << ' foo'
=> "1 foo"
irb(main):002:0> ActiveRecord::Base.connection.quote(true) << ' foo'
=> "1 foo foo"
irb(main):003:0> type = ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::MysqlString.new
=> #<ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::MysqlString:0x007fd40c15e018 @precision=nil, @scale=nil, @limit=nil>
irb(main):004:0> type.serialize(true) << ' bar'
=> "1 foo foo bar"
irb(main):005:0> type.cast(true) << ' bar'
=> "1 foo foo bar bar"
```

1 files changed, 1 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/mysql/quoting.rb b/activerecord/lib/active_record/connection_adapters/mysql/quoting.rb
index fbab654112..5b59e39d9f 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/quoting.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/quoting.rb
@@ -2,7 +2,7 @@ module ActiveRecord
   module ConnectionAdapters
     module MySQL
       module Quoting # :nodoc:
-        QUOTED_TRUE, QUOTED_FALSE = '1', '0'
+        QUOTED_TRUE, QUOTED_FALSE = '1'.freeze, '0'.freeze
 
         def quote_column_name(name)
           @quoted_column_names[name] ||= "`#{super.gsub('`', '``')}`"