Refs fab8bb4, fix the same bug for processing rollback transactions i…

…n line.

aiopg/sa/connection.py

@@ -185,6 +185,7 @@ def _rollback_impl(self):
             yield from cur.execute('ROLLBACK')
         finally:
             cur.close()
+            self._transaction = None
 
     @asyncio.coroutine
     def begin_nested(self):

tests/test_sa_transaction.py

@@ -428,7 +428,16 @@ def go():
             res2 = yield from conn.scalar(tbl.count())
             self.assertEqual(2, res2)
 
-            yield from tr2.commit()
+            yield from tr2.rollback()
+            self.assertIsNone(conn._transaction)
+
+            tr3 = yield from conn.begin()
+            self.assertIs(tr3, conn._transaction)
+            yield from conn.execute(tbl.insert().values(name='b'))
+            res3 = yield from conn.scalar(tbl.count())
+            self.assertEqual(2, res3)
+
+            yield from tr3.commit()
             self.assertIsNone(conn._transaction)
 
         self.loop.run_until_complete(go())