Use config.assests.js_compressor setting for save original js compressor

When I set `config.assets.js_compressor` to compressor with some custom options like `Uglifier.new(mangle: false)`  
The precompile task always get the error: `Sprockets::Error: unknown compressor: js_compressor`, there's because Sprocket::Environment will return `:js_compressor` symbol if user set compressor to specific compressor object when we want to get `Rails.application.assets.js_compressor` value, please see https://github.com/sstephenson/sprockets/blob/master/lib/sprockets/compressing.rb#L60

and when the task want to put `orignal_js_compressor` back to `env.js_compressor`, Sprocket can't find any compressor source name `:js_compressor` and throw error.

So it should be cache the `Rails.application.config.assets.js_compressor`(A real compressor object) to `original_js_compressor` not `Rails.application.assets.js_compressor`.

ref #201