Validation always defaults to "fast"

Related to https://github.com/LibraryOfCongress/bagit-python/issues/137 

It seems there is an indentation error / bug causing validation to _always_ default to fast validation — meaning checksums are not being checked — only the payload oxum.

I think I fixed it here? https://github.com/LibraryOfCongress/bagit-python/pull/174