Ensure asset source is utf-8 encoded

[AyoubEssrifi]

This is a modest fix for the issue mentioned here #498.

[brenogazzola]

Hey @AyoubEssrifi would mind explaining why you gave up on this? Propshaft has the same problem and I was considering forcing UTF-8 encoding there too, so I'm wondering you figured out something that would mean this approach is going to cause problems.

[AyoubEssrifi]

@brenogazzola For some reason, when i deleted the extension part of some Coffeescript or Coffe/ERB files in the main application.js:

Bad example:
//= require example.js.coffee.erb

Working example:
//= require example

that gets called on the manifest.js, Sprockets was able to correctly handle encoding and didn't have any issues with it. This was not mentioned anywhere on the docs.