Fix ActionView::Digestor to correctly pass format to LookupContext

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.

@dhh asked me to do a PR of this for rails too. I followed the contributing guide to the best of my abilities. Please let me know if I should improve the PR in some way.