Restore UA-aware auth header advertisement

Revert accidental removal of WWW-Authenticate headers.
Windows WebDAV clients receive Basic + Negotiate;
all other clients receive Basic only.

tests/test_files_auth.py

@@ -159,11 +159,11 @@ async def test_options_unauthenticated_allowed(client):
 
 
 @pytest.mark.asyncio
-async def test_unauthenticated_sends_no_auth_challenge(client):
+async def test_unauthenticated_sends_basic_auth_challenge(client):
     _, res = await client.request("PROPFIND", "/files/")
 
     assert res.status_code == 401
-    assert "www-authenticate" not in res.headers
+    assert res.headers.get("www-authenticate", "").lower().startswith('basic realm="cista"')
 
 
 @pytest.mark.asyncio