Json: workaroud for PHP fatal error

PHP function `json_decode` has a nasty bug which lead to **fatal error** if key starts with a NULL byte. Simple workaround for this is to pass `true` as the second parameter. Any tips how to solve this issue in `Nette\Utils\Json`?

<del>This pull contains only failing test for now.</del> 
Nice, you can take the same trick one step further – assuming that `"` is a lot more common in JSON than `\` we get `'#(?<=[^\\\\]")\\\\u0000(?:[^"\\\\]|\\\\.)*+"\s*+:#'`. Performance on [real-world JSON](http://www.json-generator.com/) is 820 μs for my original, 157 μs for your improvement and 20 μs for my using `\` as base matching character.