Make export_page API endpoint match ZAP
My review of plugins and core revealed that only the (relatively new) content_import plugin uses this endpoint. If there are other places where it is used, I couldn't find it. Pursuant to Mike's comment in the discussion, this MR brings Hubzilla in line with ZAP's zot endpoints for this feature.
"If there is a conflict between the way Hubzilla does something and the way Zap does something, as a general rule the Zap variant is the way we want to do it going forward and the other was left as-is for historical compatibility." (Mike M.)