Ensure the resolver moduler is loaded (#135)

Due to how the resolver is defined, it won't be automatically loaded locally by Elixir (unless eager module loading is turned on).

So we need to ensure that the resolver is loaded before checking if the function is exported.

Fixes #134

Author: sb8244

lib/oban/web/resolver.ex

@@ -431,7 +431,12 @@ defmodule Oban.Web.Resolver do
 
   @doc false
   def call_with_fallback(resolver, fun, args) when is_atom(fun) and is_list(args) do
-    resolver = if function_exported?(resolver, fun, length(args)), do: resolver, else: __MODULE__
+    resolver =
+      if Code.ensure_loaded?(resolver) and function_exported?(resolver, fun, length(args)) do
+        resolver
+      else
+        __MODULE__
+      end
 
     apply(resolver, fun, args)
   end