Allow/Fix precompiled assets with Propshaft pipeline

[serggl]

Purpose

rails/propshaft support was added with #1010
However it appears to only work with dynamic assets and fails with precompiled ones:

#<UncaughtThrowError: uncaught throw #<ActionView::Template::Error: undefined local variable or method `content' for an instance of WickedPdf::WickedPdfHelper::Assets::PropshaftAsset>>

Solution

Improve the original solution and use Propshaft's built-in resolver to handle gracefully both asset types

[serggl]

@unixmonkey please review 🙏🏽