Database: removed refetching all columns on ActiveRow::toArray() (BC break!)

Removed some magic ported from NotORM.
Some discussion: http://forum.nette.org/cs/13703-mozna-chyba-v-cachovani-sloupcu-u-dotazu-v-nette-database
Correct workaroudn is to use `->select('*')` to get all columns. Othrwise in current state is impossible to iterate only over fetched columns.