Optimize: Only JSON.stringify articles when storing to S3

Move JSON.stringify call inside the conditional that checks if article
needs to be stored. This prevents stringifying every article in large
feeds when most are already in Redis.

Fixes memory leak that was causing 'JavaScript heap out of memory' crashes.

Author: Arpith Siromoney

src/feeds.js

@@ -194,7 +194,6 @@ const feed = {
           article.score = score(article);
           article.feedurl = feedURI;
 
-          const body = JSON.stringify(article);
           const key = article.hash;
           const rank = article.score;
           const articleKey = `article:${key}`;
@@ -213,6 +212,8 @@ const feed = {
                   articleAddErr.log = zaddErr.message;
                   stream.emit('error', articleAddErr);
                 } else if ((oldscore === null) || (rank !== parseInt(oldscore))) {
+                  // Only stringify when we actually need to store it
+                  const body = JSON.stringify(article);
                   s3.putObject({
                     Key: key,
                     Body: body,