allow resetting the configured valkyrie index adapter
allow resetting the configured valkyrie index adapter

there was a bug causing `Hyrax.config.index_adapter=` to be a no-op if the index
adapter is already set. this patches it, allowing applications to change their
configured indexing behavior at runtime.

samvera/hyrax,samvera/hyrax,samvera/hyrax,samvera/hyrax