Minor improvements

[vbauerster]

Prevent code from fail, if img element will have 'hasOwnProperty' class name, for whatever reason. Unified undefined use, same style as used in forEach polyfill.

[stryju]

while the map being an Object instead of an Array is 👍 , all the other changes are unnecessary:

• calling Object.hasOwnProperty is potentially slower (and unnecessary - there's no gain)
• undefined to void 0 change - unnecessary, this is minifier's job
• x || (x = []) to x = x || [] change - ¯\_(ツ)_/¯

[vbauerster]

I've updated, according your recommendations.