Set application/js-sourcemap+json charset to unicode

[andresakata]

When upgrading the sprockets to the 4.x version, I started to get this exception in a specific asset of my project:

Encoding::UndefinedConversionError at "\xC3" from ASCII-8BIT to UTF-8

I found out that a custom transformer was receiving the data with an incorrect encoding when building the source map.

Registering the 'application/js-sourcemap+json with charset: :unicode solves the problem.

[chadlwilson]

Hi @andresakata - I think I have exactly the same issue, but with an SCSS asset that contains a Unicode char instead.

Would it make sense to you that the same charset: :unicode might be appropriate for the default configuration of the line after the one you changed? I might submit a PR.

[andresakata]

@chadlwilson yes, it does make sense to me!