Feature/json request

@markstory am I missing something, that's the standard header to check for accept headers - and the one [CakePHP has always checked](https://github.com/cakephp/cakephp/blob/master/lib/Cake/Network/CakeRequest.php#L645-L657) in earlier versions:

	public function parseAccept() {
		return $this->_parseAcceptWithQualifier($this->header('accept'));
	}

    ....

	public static function header($name) {
		$name = 'HTTP_' . strtoupper(str_replace('-', '_', $name));
		if (!empty($_SERVER[$name])) {
			return $_SERVER[$name];
		}
		return false;
	}