Memoize result of JSON.parse in json_response

This might see a minor speedup in test cases that call this method many
times.

spec/support/api_helpers.rb

@@ -29,6 +29,6 @@ module ApiHelpers
   end
 
   def json_response
-    JSON.parse(response.body)
+    @_json_response ||= JSON.parse(response.body)
   end
 end