Fixed usage of JSON.stringify in debug (#400)

When debugging is disabled, calling `debug('', JSON.stringify())` will cause the data to be stringified even if debugging is disabled. The proper usage of the `debug` package requires to use `%o` or `%O` to pretty-print an object.

Author: matthieusieben

app/steps/decorateProxyReqOpts.js

@@ -14,7 +14,7 @@ function decorateProxyReqOpt(container) {
     .then(function (processedReqOpts) {
       delete processedReqOpts.params;
       container.proxy.reqBuilder = processedReqOpts;
-      debug('Request options (after processing):', JSON.stringify(processedReqOpts));
+      debug('Request options (after processing): %o', processedReqOpts);
       return Promise.resolve(container);
     });
 }