Fix filter to return content

[michaelkl]

Hi!
First of all, thank you for your filter rework!

I'm using Ruby 2.3.3 and nanoc 4.6.2. I've got problems running your filter.

Looking on my nanoc's built-in filters I found out that filter's run method should return content. So here's the patch.

[GregOnNet]

Hi @michaelkl,

I am so sorry. I did not see your PR until now. 😓
Do you still have a need that I merge your PR? 😇

[michaelkl]

Hi @GregOnNet,

Honestly, I haven't been using it for a long time. :-)

But as I remember, it was not working for me at all (and it seemed strange). I guess it is still a good idea to make it work finally, and later is better than never.