the "Test Active Port" functionality doesn't honor listen_interface parameter

[errge]

The test_listen_port function in core.py just calls "from twisted.web.client import getPage" without specifying listen_interface as bindAddress. Therefore in a multi ISP setting, where deluge beautifully works the thanks to the listen_interface setting, the testing functionality warns the user that the port is not open, while in reality it is.

I'm not attaching a patch, because the fix is unfortunately not very simple, since the simple getPage twistedweb API is not twisted enough to support bindAddress, only the more complicated (twisted?) Agent APi supports that functionality. So either someone has to add bindAddress to getPage or rewrite this test_listen_port function to use twisted.web.client.Agent.