add default value to image srcs

app/comic-book.js

@@ -3,7 +3,7 @@ let LoadIndicator = require('./view/load-indicator')
 
 module.exports = class ComicBook extends EventEmitter {
 
-  constructor (srcs) {
+  constructor (srcs = []) {
     super()
 
     // requested image srcs

test/comic-book.test.js

@@ -34,7 +34,7 @@ describe('ComicBook', function () {
       let comic = new ComicBook(srcs)
 
       comic.on('preload:start', function () {
-        assert(true)
+        assert(true, 'start event should have been emitted')
         done()
       })