Fix Tags String Parsing Problem

See this issue: https://github.com/tumblr/pytumblr/issues/15.

Code:

    client.create_photo(blogName, source='http://i.imgur.com/l6PpCfK.jpg', tags='dog,corgi,puppy')

Old:

![screen shot 2013-07-28 at 1 21 57 pm](https://f.cloud.github.com/assets/164822/867851/23c315be-f746-11e2-967a-fb5f35ec40d2.jpg)

New:

![screen shot 2013-07-28 at 2 09 00 pm](https://f.cloud.github.com/assets/164822/867876/3b13e814-f74c-11e2-8b42-e44a579fd937.jpg)