There is a slight tension between how template rendering uses `finder`: its "registered_details" (such as `formats`, `variants`, `locale` etc.) are pretty much constant and unchanging through out the whole request, therefore the `#find` method was mainly meant to accept template name, view prefixes and whether its a partial template.

But looking at the code, I can't figure out why passing `:formats` and `:variants` doesn't work, it looks to be supported.

PS: duping the finder seems to work, but purely by accident: `formats` and `variants` are stored in `@details` i-var, which is a Hash, so after duping, the new `finder` still uses the same `@details` Hash as the old one, but details setters (`finder.formats=`) dup the `@details` Hash on writes, so it actually works.