[session] generated xml does not have `prove=true` for proved goal
Steps to reproduce :
- generate a proof session with
why3find prove -s foo.mlw
- open the session in ide with
why3 ice -L . foo
and save it - diffs the xml and xml.bak : attribute
proved=true
has been set on every node
This is a problem when generating an HTML file from the session file using why3 session html foo