Make default to @prepared_statements = false in Mysql2Adapter

Prepared statements in mysql2 still have GC issue.
brianmario/mysql2#694

activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb

@@ -37,6 +37,7 @@ class Mysql2Adapter < AbstractMysqlAdapter
 
       def initialize(connection, logger, connection_options, config)
         super
+        @prepared_statements = false unless config.key?(:prepared_statements)
         configure_connection
       end
 

activerecord/test/cases/bind_parameter_test.rb

@@ -31,7 +31,8 @@ def setup
       ActiveSupport::Notifications.unsubscribe(@subscription)
     end
 
-    if ActiveRecord::Base.connection.supports_statement_cache?
+    if ActiveRecord::Base.connection.supports_statement_cache? &&
+       ActiveRecord::Base.connection.prepared_statements
       def test_bind_from_join_in_subquery
         subquery = Author.joins(:thinking_posts).where(name: 'David')
         scope = Author.from(subquery, 'authors').where(id: 1)