Improve the registration of the Engines' assets path in propshaft

[rafaelfranca]

I'm an not making the mistake to commit this without someone trying it.

@dhh or @rmacklin can you check if this works in the same way as before?

[rmacklin]

With the integration test from #20, this change appears to pass CI - it doesn't break in the way that 66ce0f8 broke the test suite (#41)

[brenogazzola]

I've tested it against a production app already running Propshaft and the output of rake assets:reveal is identical, so it seems its correctly picking the assets from gems.

[rafaelfranca]

Thanks! @rmacklin can you open a new PR extracting the integrations tests from #20?

[rmacklin]

@rafaelfranca Done: #44