Make helm-marked-candidates always return candidates

Currently, the "Find Files" action differs in behavior on sources (with type `file`) that have a `coerce` property set, between the cases when you select one file, and when you select more than one file.

If you select more than one file, Find Files will correctly open the coerced ("real") files, but if you Find Files with only one file selected, it will try to open the display value of the file, without coercing first. 

`helm-marked-candidates` seems to be the right place to apply the coercion for the single candidate also, but superficially reading the helm code, there seems to be more than one place where the display value (and not the coerced/real candidate value) is being used.

I hope you'll consider this patch for inclusion anyway (-: