Added variable to track the downloaded content size in response

lib/collection/response.js

@@ -255,7 +255,12 @@ _.assign(Response.prototype, /** @lends Response.prototype */ {
              * @private
              * @type {Number}
              */
-            responseSize: stream && stream.byteLength
+            responseSize: stream && stream.byteLength,
+
+            /**
+             * @type {Number}
+             */
+            downloadedBytes: options.downloadedBytes
         });
     }
 });
@@ -406,7 +411,8 @@ _.assign(Response.prototype, /** @lends Response.prototype */ {
         var sizeInfo = {
                 body: 0,
                 header: 0,
-                total: 0
+                total: 0,
+                downloadedBytes: this.downloadedBytes
             },
 
             contentEncoding = this.headers.get(CONTENT_ENCODING),
@@ -441,6 +447,10 @@ _.assign(Response.prototype, /** @lends Response.prototype */ {
                 this.body.toString().length;
         }
 
+        if(!sizeInfo.downloadedBytes) {
+            sizeInfo.downloadedBytes = sizeInfo.body;
+        }
+
         // size of header is added
         // https://tools.ietf.org/html/rfc7230#section-3
         // HTTP-message   = start-line (request-line / status-line)
@@ -452,7 +462,7 @@ _.assign(Response.prototype, /** @lends Response.prototype */ {
             this.headers.contentSize();
 
         // compute the approximate total body size by adding size of header and body
-        sizeInfo.total = (sizeInfo.body || 0) + (sizeInfo.header);
+        sizeInfo.total = (sizeInfo.downloadedBytes || sizeInfo.body || 0) + (sizeInfo.header);
 
         return sizeInfo;
     },