respect rails 5 assigns render option

[dleavitt]

Rails 5 supports an assigns option to render/render_to_string that allows setting instance variables available to the template. This is particularly handy for rendering outside of controllers where these instance variables wouldn't otherwise be available.

It's a pass-through so I didn't add any new tests (but could.)

[unixmonkey]

Thank you very much. There are other options that probably need passed through (like :inline from #797), or perhaps we could take a reverse approach and only peel off options we need to mess with (as suggested in #633), but this does the job great. Thank you!

[dleavitt]

thanks!