Adds fix for content-lenght value missing for files uploaded via ActionD...

...ispatch:Http::UploadedFile
@jcoyne Although I like the `payload.is_a?` approach better than using `payload.class.name == "ActionDispatch::Http::UploadedFile"` that problem with `payload.is_a?` is that it will force a dependency between ActiveFedora and ActionDispatch::Http::UploadedFile and the code won't run unless the class ActionDispatch::Http::UploadedFile is defined. 

I am thinking of reverting back to `class.name == "ActionDispatch::Http::UploadedFile"`. Thoughts?