fix: return the pool from _async_init__ if it's already initialized (#1104)

Author: guacs

asyncpg/pool.py

@@ -396,7 +396,7 @@ def __init__(self, *connect_args,
 
     async def _async__init__(self):
         if self._initialized:
-            return
+            return self
         if self._initializing:
             raise exceptions.InterfaceError(
                 'pool is being initialized in another task')