Merge pull request #82 from moteus/master

Fix. Lua versino of `multi::add_handle` returns error same as C.

Author: moteus

src/lua/cURL/impl/cURL.lua

@@ -592,7 +592,9 @@ function Multi:add_handle(e)
   assert(self._easy.n >= 0)
 
   local h = e:handle()
-  if self._easy[h] then return self end
+  if self._easy[h] then
+    return nil, curl.error(curl.ERROR_MULTI, curl.E_MULTI_ADDED_ALREADY or curl.E_MULTI_BAD_EASY_HANDLE)
+  end
 
   local ok, err = add_handle(self, h)
   if not ok then return nil, err end