Ensure threading._shutdown gets patched too

Since threading can get imported multiple times, ensure that the _shutdown
symbol is patched so the _MainThread matches what is in _active.

This patch fill fix the KeyError on exit under Python 2.7

eventlet/green/threading.py

@@ -6,7 +6,7 @@ from eventlet.support import greenlets as greenlet
 
 __patched__ = ['_start_new_thread', '_allocate_lock', '_get_ident', '_sleep',
                'local', 'stack_size', 'Lock', 'currentThread',
-               'current_thread', '_after_fork']
+               'current_thread', '_after_fork', '_shutdown']
 
 __orig_threading = patcher.original('threading')
 __threadlocal = __orig_threading.local()

tests/patcher_test.py

@@ -383,6 +383,14 @@ print len(threading._active)
         self.assert_(lines[0].startswith('<_GreenThread'), lines[0])
         self.assertEqual(lines[1], "1", lines[1])
 
+    def test_keyerror(self):
+        new_mod = """import eventlet
+eventlet.monkey_patch()
+"""
+        self.write_to_tempfile("newmod", new_mod)
+        output, lines = self.launch_subprocess('newmod')
+        self.assertEqual(len(lines), 1, "\n".join(lines))
+
 
 class GreenThreadWrapper(ProcessBase):
     prologue = """import eventlet