use let instead of var

Author: mcollina

lib/request.js

@@ -93,7 +93,7 @@ function Request (options) {
   })
 
   // we keep both payload and body for compatibility reasons
-  var payload = options.payload || options.body || null
+  let payload = options.payload || options.body || null
   if (payload && typeof payload !== 'string' && !(typeof payload.resume === 'function') && !Buffer.isBuffer(payload)) {
     payload = JSON.stringify(payload)
     this.headers['content-type'] = this.headers['content-type'] || 'application/json'