This reverts commit 356a367eb5.
We can't use headers_sent() to determine an accurate
content length, since PHP defaults to buffering a couple of bytes
even without ob_start() (see "output_buffering" setting).
This makes the patch harmful, since it breaks any responses relying
on more structure data, like removing closing brackets from JSON.
Which in turn breaks the CMS in horrible ways (see #8010).
See #7574 for context.
These APIs are primarily intended to let developers write custom 404 handlers. They can define an onBeforeHTTPError404() method on an Extension that gets added to Controller or RequestHandler.
The SS_HTTPResponse_Exception object has also been tidied up to override the status info of any SS_HTTPResponse object that might get passed. This is mainly to make it easier for callers (such as ContentController and ModelAsController) to use RequestHandler::httpError() more consistently.
