Fix the looping issue on the HTTP request

There is an issue where the HTTP request is multiplied by the number of properties to scrape, because the call to curlrequest is inside `for(var key in config.scrape){`.

Additionally, when there are several properties to scrape and one of them doesn't have an `extract`, the promise is correctly rejected but the script tries to scrape the url anyway as soon as it loops over a correctly formatted property, throwing an error on `mapping["extract"].toLowerCase()`.