Aloha!

As we are converting '1' to true, i think we should convert '0' to false. I'm not quite sure if this is a "bug" or an unexpected behaviour. What do you guys think? Can i open a PR for https://github.com/leberknecht/symfony/tree/fix-bool-to-string-transformer ? And if so, does this break BC if we change that behaviour? Aloha!