fix dump_session(byref=True) bug when the multiprocessing module …

…was imported

Just calling `import multiprocessing` creates a `'__mp_main__'` entry in `sys.modules` that is simply a reference to the `__main__` module. In the old version of the test for module membership, objects in the global scope are wrongly attributed to this virtual `__mp_main__` module. And therefore `load_session()` fails. Testing for object identity instead of module name resolves the issue.

dill/_dill.py

@@ -412,7 +412,7 @@ def _module_map():
 def _lookup_module(modmap, name, obj, main_module): #FIXME: needs work
     """lookup name if module is imported"""
     for modobj, modname in modmap[name]:
-        if modobj is obj and modname != main_module.__name__:
+        if modobj is obj and sys.modules[modname] is not main_module:
             return modname
 
 def _stash_modules(main_module):