Deletions: don't purge keys so Delete/Undo activities can be signed

lib/pleroma/user.ex

@@ -1680,8 +1680,6 @@ def purge_user_changeset(user) do
       email: nil,
       name: nil,
       password_hash: nil,
-      keys: nil,
-      public_key: nil,
       avatar: %{},
       tags: [],
       last_refreshed_at: nil,

test/pleroma/user_test.exs

@@ -1651,8 +1651,8 @@ test "delete/1 purges a user when they wouldn't be fully deleted" do
              email: nil,
              name: nil,
              password_hash: nil,
-             keys: nil,
-             public_key: nil,
+             keys: "RSA begin buplic key",
+             public_key: "--PRIVATE KEYE--",
              avatar: %{},
              tags: [],
              last_refreshed_at: nil,