Allow updating of the httpsRedirect attribute for loadbalancers.

According to the Rackspace docs, the httpsRedirect and halfClosed
attributes are updatable via the API: http://bit.ly/J7D5yg
Adding halfClosed as an updatable attribute.
Merge pull request #40 from nstielau/allow_updating_httpsRedirect

Allow updating of the httpsRedirect attribute for loadbalancers.

This should be fine, just be aware that you're not error checking it in cloudlb/loadbalancers.py so if you try to set httpRedirect on a tcp load balancer it will likely break.