Defer aborting requests when settings src to a cached URL

[zcorpan]

See #261 (comment)

A design goal of the "update the image data" algorithm is that it should not matter in which order a script does something (modulo Web compat). However, when setting src to a cached URL, the spec says to abort the current and pending requests. If the script then goes on to add a srcset attribute or insert the image to a picture, maybe the aborted pending request becomes the selected candidate. So it would be nice if we could avoid aborting it sync.