Add return true to prefetch

[timneutkens]

Fixes '#540'

This avoids unintended bugs because you'd expect the function to return something instead of 'void'

[nkzawa]

We wouldn't like to return true here. I think void rather makes sense, or a promise which is resolved when prefetching finished ?

[timneutkens]

Makes sense, will change it

[arunoda]

@nkzawa I think it's kind a hard to identify prefetching finished event with the current setup.
We may need to do few more stuff.
But I am not sure, that's something worthy to do.

[arunoda]

I think we could fix #540 without changing anything here.

[timneutkens]

@arunoda fair enough, will change the example then 👍

[arunoda]

Thanks @timneutkens
I think we already landed a fix :)
#563