fix(middleware): does not work with mootools (#2591)

Change karma.js to check for own property during for...in loop
Because mootools adds functions to the array prototype,
the use of for..in will get more than the array elements.

lib/middleware/karma.js

@@ -179,6 +179,10 @@ var createKarmaMiddleware = function (
             var filePath = file.path
             var fileExt = path.extname(filePath)
 
+            if (!files.included.hasOwnProperty(i)) {
+              continue
+            }
+
             if (!file.isUrl) {
               filePath = filePathToUrlPath(filePath, basePath, urlRoot, proxyPath)