Make File's lastModified return the current time as the default
Make File's lastModified return the current time as the default

This is per spec (https://w3c.github.io/FileAPI/#dfn-lastModified) and allows us to pass the updated XMLHttpRequest/FormData-append.html test.

Zirro/jsdom,mzgol/jsdom,tmpvar/jsdom,snuggs/jsdom,fiftin/jsdom,mzgol/jsdom,tmpvar/jsdom,Zirro/jsdom,mzgol/jsdom,fiftin/jsdom,fiftin/jsdom,Joris-van-der-Wel/jsdom,Joris-van-der-Wel/jsdom,Joris-van-der-Wel/jsdom,snuggs/jsdom