Apply sprocket helpers refactor to 3-1-Stable

[lardawge]

These commits were never applied to 3-1-stable and were causing uninitialized constant Sprockets::Helpers when running rake assets:precompile. All tests passing and browser tested. This would invalidate pull request #1387.

[lardawge]

@wycats, Can we resolve this? I believe I corralled all the commits leading to this change. Either this needs to be applied or the referenced pull request needs to be applied for rake assets:precompile to work in 3-1-stable.

/cc @dhh

[sikachu]

I'm closing #1387 because you said you want to invalidate that.

Assign to @dhh for comment, and put to 3.1 milestone.

[lardawge]

Closing because it has been committed by wycats