[WIP] Pass context to renderer

[Borzik]

This PR is based on a fix from #150

It adds passing context to renderer, so we can call image_path, asset_path, and other helpers again.

[AlexRiedler]

I think this makes sense; is there docs about this ? cause this only affects Sprockets 4+ right ? or am I mistaken ?

[Borzik]

Sorry for such a small description, my main purpose was to get someone's attention to that fix and issue itself (I've faced it). I thought that since this issue was fixed for me this way, it should work for everyone, but now I realize the problem is deeper.

I am using Sprockets v 3.6, not Sprockets 4. But for some reason HandlebarsProcessor is being used for rendering, not HandlebarsTemplate class, although Sprockets 3 HandlebarsTemplate is installed.

For more information, please check this reduced test case. I've tried to do it as small as possible, only one template, single view, no dependencies. All versions (both working and failing) are added to Gemfile, so you can see what works for which versions (by commenting/uncommenting some parts of code).

[Borzik]

Actually the problem is much simpler: We are testing that Sprockets version is less than 3, but should test if it's less than 4. Created another PR for that.