prevent leaking "error" to global scope

lib/http_client.js

@@ -115,7 +115,7 @@ client.prototype.request = function(method, path, params, callback){
         });
 
         res.on('end', function(){
-            error = null;
+            var error = null;
             try{ data = JSON.parse(data); }catch(e){}
             if(data['errors']){
                 error = data['errors'];