This api call to the Weather Underground service works as expected using MRI ruby-1.9.3-p429 but not when using MRI ruby-2.0.0-p195.
This gist shows the code and the response when using 1.9.3 and 2.0.0.
The code sample is taken directly from the Weather Underground's documentation
Update
After working with @zph and @injekt there have been some interesting findings. I am including wunderground-working-2_0.rb as an example of a working version of the Weather Underground API using Ruby MRI 2.0.0.
The interesting piece was when @injekt showed that the rest-client gem could call the api without error. He then found that the gem was manually rescuing the Zlib::DataError (https://github.com/rest-client/rest-client/blob/master/lib/restclient/request.rb#L258).
So, for now, if I want to use the Weather Underground API and MRI 2.0.0, I'll be doing it with the rest-client gem.
Additionally interesting reading can be found with this Ruby core commit, where a significant update was made to the Zlib libraries.
Troubleshooting Repo related to https://gist.github.com/geopet/5782836