Cast HTTP response body for JSON to string (regression from #4) (#5)

Oj uses `readpartial` instead of `to_s` if the value given for the arguments has a `readpartial` method.
Also, HTTP.rb splits the response if `Transfer-Encoding: chunked` is returned in the HTTP headers.

Will use `to_s` like `JSON.parse` which was used previously.

lib/goldfinger/result.rb

@@ -74,7 +74,7 @@ module Goldfinger
     end
 
     def parse_json
-      json = Oj.load(@body, mode: :null)
+      json = Oj.load(@body.to_s, mode: :null)
 
       @subject    = json['subject']
       @aliases    = json['aliases'] || []