Allow 0 as key in the pool

Close #848

lib/pool.py

@@ -95,11 +95,11 @@ class AbstractConnectionPool(object):
         """Put away a connection."""
         if self.closed:
             raise PoolError("connection pool is closed")
+
         if key is None:
             key = self._rused.get(id(conn))
-
-        if not key:
-            raise PoolError("trying to put unkeyed connection")
+            if key is None:
+                raise PoolError("trying to put unkeyed connection")
 
         if len(self._pool) < self.minconn and not close:
             # Return the connection into a consistent state before putting