Fix the PutRelativeFile behavior

Currently, when PutRelativeFile is called from the Excel > Save As function (only place where it can be observed), a new document is created as a copy of the document hosting the saved file in the same place.

This might not be what we want:

• If I edit the main file test.xlsx of a document with some attached documents, it will create another document with a copy of the main file (OK) but also all the attachments and the metadata . Only changes are the title of the newly created document and the filename of the blob saved as <new_name>.
• If I edit an attachement, it will do the same! And the copied doc will have the name of the copied attachment as a title...

-----------------------------------
Let's;

• Throw an exception when trying to Save As and return 501 Not Implemented.
• Inform Microsoft that we only support PutRelativeFile for binary conversion.
• Document that Save As is not available.