fix(config) Avoid php to handle the config key for 404 pages as int

[gmutschler]

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[gmutschler]

@Log1x : Anyhow PHP inteprets config['404'] as config[404]. Weirdly that's the first time I come across this issue and it may be an issue with only some PHP versions/confs. But better safe than sorry as we say.

I found some related issues on SO, tending to confirm this observation:

• https://stackoverflow.com/questions/4100488/a-numeric-string-as-array-key-in-php
• https://stackoverflow.com/questions/3445953/how-can-i-force-php-to-use-strings-for-array-keys