Validate twitter agent credentials

Added variable caching and validation of credentials. Would love to get the specs passing but for some reason I'm getting a NoMethodError on allow_any_instance_of which is bizarre is rspec 2.14 has this available. Open to other ways of doing this or advice if I'm doing something wrong. 
On one hand I agree with you, the cache in the #credentials  method makes this less useful. But when you make the decision not to cache here because of that, you're making an assumption about how another class behaves which could at some point change. For this one it's probably unlikely to change and not memoizing here is pretty safe. However writing this method with no assumptions about how methods in Agent behave seems slightly better to me.