Put all successful responses in Hashie::Mash

Before only responses with code == 200 were being put in a Hashie::Mash. However Yesmail can respond with 202, 204, etc codes for successful requests. These will all be handled just like a 200.
Merge pull request #6 from apartmentlist/feature/2xx-response-codes

Put all successful responses in Hashie::Mash