Added .setResponseType handling

Suitable for e.g. arraybuffers requests. Takes standard values, as in goog.net.XhrIo.ResponseType
Example: 
```clojure
(http/get "http://example.com/some-binary-api" 
                              {:with-credentials? false 
                               :response-type "arraybuffer"})
```
Defaults to "" 

In the returned map, adds :response-type key, which returns (.getResponseType target) for response consistency checking and 
 :response, which returns (.getResponse target), the proper way of handling non-default request types.