Add option to hide the X-Commerce-Core HTTP header

Describe your bug or feature request.

Hi there!

In the #3220338 improvement add the "X-Commerce-Core: 1" HTTP header.
For D8 or above we can hide the HTTP header with alter an event dispatcher. But for the D7 it can be solved a little bit more difficult and with overheads (use hook_module_implements_alter and just exclude a hook_init from the commerce.module file).

Will be better if we have a Drupal variable and call the drupal_add_http_header function inside hook_init with a condition.