Keep ostatus id as activity id.

lib/pleroma/web/ostatus/ostatus.ex

@@ -63,6 +63,7 @@ def handle_note(entry, doc \\ nil) do
     to = to ++ mentions
 
     date = string_from_xpath("/entry/published", entry)
+    id = string_from_xpath("/entry/id", entry)
 
     object = %{
       "type" => "Note",
@@ -81,7 +82,7 @@ def handle_note(entry, doc \\ nil) do
       object
     end
 
-    ActivityPub.create(to, actor, context, object, %{}, date)
+    ActivityPub.create(to, actor, context, object, %{"id" => id}, date)
   end
 
   def find_or_make_user(uri) do

test/web/ostatus/ostatus_test.exs

@@ -7,6 +7,7 @@ test "handle incoming note - GS, Salmon" do
     {:ok, [activity]} = OStatus.handle_incoming(incoming)
 
     assert activity.data["type"] == "Create"
+    assert activity.data["id"] == "tag:gs.example.org:4040,2017-04-23:noticeId=29:objectType=note"
     assert activity.data["object"]["type"] == "Note"
     assert activity.data["published"] == "2017-04-23T14:51:03+00:00"
     assert activity.data["context"] == "tag:gs.example.org:4040,2017-04-23:objectType=thread:nonce=f09e22f58abd5c7b"