Fix crash on Document.save() if conflict but not _rev

[adrienverge]

A common pattern is to try to save a document, and pass if it exists:

try:
    await Document(db, "id").save()
except aiocouch.exception.ConflictError:
    pass

Currently, in case of conflict, aiocouch fails with:

[...]
aiohttp.client_exceptions.ClientResponseError: 409...
During handling of the above exception, another exception occurred:
[...]
KeyError: 'rev'

I propose to set rev = None in error handling, if not available.
Another solution would be to always define Document._data["_rev"], but
this has other implications.

[bmario]

Thanks for pointing this issue out. Just a few small changes.

[adrienverge]

Thanks for your review, PR updated with:

--- a/aiocouch/exception.py
+++ b/aiocouch/exception.py
@@ -62,11 +62,11 @@ def raise_for_endpoint(endpoint, message, exception, exception_type=None):
 
     message_input = {}
 
-    with suppress(KeyError, AttributeError):
+    with suppress(AttributeError):
         message_input["id"] = endpoint.id
         message_input["endpoint"] = endpoint.endpoint
         message_input["rev"] = endpoint._data.get("_rev")
-    with suppress(KeyError, AttributeError):
+    with suppress(AttributeError):
         message_input["document_id"] = endpoint._document.id
         message_input["document_rev"] = endpoint._document._data.get("_rev")
 
--- a/tests/test_document.py
+++ b/tests/test_document.py
@@ -59,9 +59,9 @@ async def test_conflict(filled_database):
         await doc2.save()
 
 
-async def test_conflict_without_rev(filled_database):
-    doc1 = Document(filled_database, "fou")
-    doc2 = Document(filled_database, "fou")
+async def test_conflict_without_rev(database):
+    doc1 = Document(database, "fou")
+    doc2 = Document(database, "fou")
 
     doc1["blub"] = "new"
     await doc1.save()