Asset Encoding Negotiation

Part of #569.

cc @nevir cause you seemed interested.
Wanted to use "encoding" for any content encoding used.

asset.charset is used for Content-Type charset and asset.encoding is used for Content-Encoding.

> On Jul 3, 2014, at 4:13 PM, Jeremy Kemper <notifications@github.com> wrote:
> 
> In lib/sprockets/processing.rb:
> 
> > @@ -193,7 +193,7 @@ def process(processors, filename, load_path, name, content_type, data)
> >  
> >        {
> >          source: data,
> > -        encoding: data.encoding,
> > +        charset: data.encoding.name.downcase,
> encoding -> charset?
> 
> —
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