Convert all response values to a string

Although RestClient::Response acts like a string, it raises a warning
if #to_i is called on it, such as by our own Util.to_numeric (since
RestClient::Response used to have behavior of returning an HTTP status
code). We now start with response values as strings for less
confusing processing by clients.

Author: ianfleeton

lib/paypal/nvp/request.rb

@@ -50,7 +50,7 @@ def post(method, params)
       def handle_response
         response = yield
         response = CGI.parse(response).inject({}) do |res, (k, v)|
-          res.merge!(k.to_sym => v.first)
+          res.merge!(k.to_sym => v.first.to_s)
         end
         case response[:ACK]
         when 'Success', 'SuccessWithWarning'