Fix fetching post from another takahe by searching its url (#661)

activities/services/search.py

@@ -82,7 +82,11 @@ class SearchService:
         if response.status_code >= 400:
             return None
         content_type = response.headers.get("Content-Type", "").lower()
-        if content_type not in ["application/json", "application/ld+json"]:
+        if content_type not in [
+            "application/json",
+            "application/ld+json",
+            "application/activity+json",
+        ]:
             return None
         document = canonicalise(response.json(), include_security=True)
         type = document.get("type", "unknown").lower()