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, 16 insertions, 0 deletions

diff --git a/activerecord/test/cases/quoting_test.rb b/activerecord/test/cases/quoting_test.rb
index c01c82f4f5..225e23bc83 100644
--- a/activerecord/test/cases/quoting_test.rb
+++ b/activerecord/test/cases/quoting_test.rb
@@ -149,5 +149,21 @@ module ActiveRecord
         assert_equal "1800", @quoter.quote(30.minutes)
       end
     end
+
+    class QuoteBooleanTest < ActiveRecord::TestCase
+      def setup
+        @connection = ActiveRecord::Base.connection
+      end
+
+      def test_quote_returns_frozen_string
+        assert_predicate @connection.quote(true), :frozen?
+        assert_predicate @connection.quote(false), :frozen?
+      end
+
+      def test_type_cast_returns_frozen_value
+        assert_predicate @connection.type_cast(true), :frozen?
+        assert_predicate @connection.type_cast(false), :frozen?
+      end
+    end
   end
 end