Fix lookup of templates with specific formats

Hi, I was playing around trying to use cache_digests to generate a checksum for a generated pdf (based on views with pdf as the format).

After much head-scratching I think I actually found a small bug in TemplateDigestor. There seems to be a missing argument in the call to ActionView::LookupContext which causes formats of templates to be ignored.

After this small change suddenly my digests started updating as I modified partial used by the pdf.