Make on_reconnect callback functional

Previously, if the socket.io connection got interrupted and reconnected
the on_connect callback got called again. I noticed that there is a
on_reconnect function to overide in the base namespace class, but since
there is no socket.io command corresponding to on_reconnect it never
gets called.

This commit adds a check to see if we have been connected before, and if
so, calls the on_reconnect callback instead of the on_connect callback agin.