Add ostatus conversation as context.

lib/pleroma/web/ostatus/ostatus.ex

@@ -41,7 +41,12 @@ defmodule Pleroma.Web.OStatus do
     [author] = :xmerl_xpath.string('/entry/author[1]', doc)
     {:ok, actor} = find_or_make_user(author)
 
-    context = ActivityPub.generate_context_id
+    context = string_from_xpath("/entry/ostatus:conversation[1]", doc) |> String.trim
+    context = if String.length(context) > 0 do
+      context
+    else
+      ActivityPub.generate_context_id
+    end
 
     to = [
       "https://www.w3.org/ns/activitystreams#Public"

test/web/ostatus/ostatus_test.exs

@@ -9,6 +9,7 @@ defmodule Pleroma.Web.OStatusTest do
     assert activity.data["type"] == "Create"
     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"
   end
 
   describe "new remote user creation" do