[PR] DateTime RFC7231 constant

Good early morning (late night for me) internals! I would like to propose a small addition be made to the DateTime date format constant definitions. https://github.com/php/php-src/pull/882 This is my first contribution to PHP's core. I wasn't sure if something of this nature would require an RFC, but I tried to register for a Wiki account and was met with page errors anyway (if anyone could point me in the right direction to notify someone of the wiki registration page failing <https://wiki.php.net/rfc?do=register>, that'd be awesome). Since I was unable to create an RFC anyway, I attempted to make the pull request description as close to the RFC template/format as I could. I'm sorry if I'm missing anything there. Please let me know if I am. My C skills are near non-existent, but luckily this PR requires very little C code. :P Thank you all for your time! - Trevor