Move order submission response from header to actual content in ajax.php

[diptobi1]

Currently, when a users saves their orders, the server replies to the ajax request with a response, which is completely written into a customized HTTP-header instead of the actual content.

This method is nowadays deprecated (https://tools.ietf.org/html/rfc6648) and has as one drawback that the header size is usually limited to small data sizes (~ 8 KB) by default server configurations. Though, as the response also includes the order data, custom variants where users might get many units (~100) are just scraping at the 8 KB size limit.

It is probably the cleanest and not too complicated to just move the ajax-json-response to the actual content of the response and use a standard application/json content header as it is the common practice.