cordova prepare misses updated files with same size and older timestamp

[brody4hire]

As reported by @knight9999 on Slack: if a file in www is updated to have the same size but an older timestamp compared to the corresponding file within platforms then cordova prepare will miss the file. Reproduction is given in: https://github.com/knight9999/sampleUnupdate

I think the best solution would be to check the md5 sum whenever the file in www and the corresponding file in platforms have the same size.