BUG: cannot change polling period after it was set to 0 once
I found, that after the polling period wsa once set to 0, it cannot be set back to any another value:
```py
In [1]: from tango import DeviceProxy
In [2]: dev = DeviceProxy("sys/tg_test/1")
In [3]: dev.poll_attribute("short_scalar", 100)
In [4]: dev.get_attribute_poll_period("short_scalar")
Out[4]: 100
In [5]: dev.poll_attribute("short_scalar", 0)
In [6]: dev.get_attribute_poll_period("short_scalar")
Out[6]: 0
In [7]: dev.poll_attribute("short_scalar", 100)
In [8]: dev.get_attribute_poll_period("short_scalar")
Out[8]: 0
```
But, if you ten set it in Jive - it works again
issue