Bug fix and a change in OAuth Authorization parameter quoting

Hi John - a couple changes here. The one to add .to_s to method seems critical to get the tool working at all on my 10.8 Mac. Don't know if you've run into this and already addressed it in a local copy or not. The second thing is just a step I took to make the behavior of the tool more closely match the Tumblr iOS app in it specification of OAuth parameters in the Authorization header. I am still trying to track down why I can't get authorization with my tokens to work through the tumblr_client irb console.