Prevent "remove_mapping" in ElasticSearch::Api::Index#delete when id is nil

I was having this problem with a race condition during re-index operations with Escargot would send a nil instead of a valid id to the delete command.  I fixed the issue in my fork of Escargot, but it seems like Rubberband would benefit from a fix for this unexpected behavior as well.

I wrote a spec for my fix in Rubberband and just return a false when id is nil to prevent trashing the whole index.