right sqlite datetime format string with test case

fixed issue #276

PHPUnit results:

```
C:\wamp\bin\php\php5.4.3\php.exe C:\Users\Boris\AppData\Local\Temp\ide-phpunit.php --no-configuration --filter "/::test_datetime_to_string( .*)?$/" SqliteAdapterTest C:\repos\php-activerecord\test\SqliteAdapterTest.php
Testing started at 11:38 ...
(Logging SQL queries disabled, PEAR::Log not found.)
(Cache Tests will be skipped, Memcache not found.)
PHPUnit 3.7.19 by Sebastian Bergmann.



Time: 3 seconds, Memory: 3.25Mb

OK (1 test, 1 assertion)

Process finished with exit code 0
```