Don't override user-agent header if it's been set

lib/pleroma/http/request_builder.ex

@@ -34,10 +34,12 @@ def url(request, u), do: %{request | url: u}
   @spec headers(Request.t(), Request.headers()) :: Request.t()
   def headers(request, headers) do
     headers_list =
-      if Pleroma.Config.get([:http, :send_user_agent]) do
+      with true <- Pleroma.Config.get([:http, :send_user_agent]),
+           nil <- Enum.find(headers, fn {key, _val} -> String.downcase(key) == "user-agent" end) do
         [{"user-agent", Pleroma.Application.user_agent()} | headers]
       else
-        headers
+        _ ->
+          headers
       end
 
     %{request | headers: headers_list}