Make MysqlAdapter.table_exists? to respect the mysql configuation lower_case_table_names

MysqlAdapter.table_exists? method doesn't work well when mysql is configured with lower_case_table_names = 1 in my.cnf.

This causes the following code to fail when running the command rake test on the rails branch from the second time onwards.

rails/activerecord/test/schema.rb

  create_table "CamelCase", :force => true do |t| t.string :name ;  end

Reason: Since, we are passing the :force option to the create_table method, it will try to first drop the table if exists. But table_exists?('CamelCase') will return false though we already have the table created when we ran the test for the first time. As it returned false, we directly create the table without even dropping it. This will raise the 'Table already exists' excpetion.