Set state to processing for Peppol edocument before actually process them

The actual process method checks that the state of the document is still
processing so it must be written before and not rely on the transition to write
it.

Closes #14626

modules/edocument_peppol/edocument.py

@@ -207,6 +207,8 @@ class Peppol(Workflow, ModelSQL, ModelView):
     @Workflow.transition('processing')
     def process(cls, documents):
         cls.lock(documents)
+        # write state before calling _process
+        cls.write(documents, {'state': 'processing'})
         for document in documents:
             if document.direction == 'out':
                 cls.__queue__._process(document)