Make environment version affect digest of assets with no preprocessor

[rafbm]

Disclaimer: I did this pretty quickly with no prior knowledge of Sprockets internals.

I followed the logic of #404 and created an equivalent test in test_environment.rb using "blank.gif" instead of "gallery.js", which was failing. Then I realized that PR clearly covered the if processors.any? branch only and not the else. I implemented what seemed to be the closest and most inoffensive change (doesn’t affect the implementation of file_digest or similar), which got my test to pass. Then I blindly fixed the failing tests in test_asset.rb, and here we are.

[rafaelfranca]

This was closed by 5038ad5. I just applied your test in 7f64817 to make sure it was fixed.

[rafbm]

Awesome! Thanks much.