The document sheet specified in the URL is ignored by the office export and the old PDF export

It seems that we have a regression on newer versions of XWiki. The Document sheet is not taken into account by the export action, so, for example if you want to export a page that is rendered using a document sheet (in old PDF, RTF, ODT formats) the exported document will not take in consideration the sheet during the rendering of the document.

Steps to reproduce :

• XWiki version : 14.10.13 or 15.7
• Create a simple sheet page (Sandbox.Sheet), put the string "Hello World !" as content
• Open the XWiki home page (Main.WebHome) using the sheet (http://localhost:8080/xwiki/bin/view/Main/?sheet=Sandbox.Sheet), you will notice that in "view" the page is rendered correctly, see screenshot :
  
• Now try to export the home page in (old) PDF or RTF with the sheet applied (url : http://localhost:8080/xwiki/bin/export/Main/?format=pdf&sheet=Sandbox.Sheet)

Expected result : the home page is exported in PDF and the PDF file contains the content generated by the sheet, in our case it should be "Hello World !". See screenshot from an 9.8.1 instance :

Current result : The home page is exported in PDF but contains the initial content of the home not the one generated by the sheet. See screenshot from a 14.10.13

Note that I did not reproduced the issue on an 11.10.5 xwiki instance.