Prohibit find() with no selector in Bulk API

RUBY-722

While we already prohibited find() with no selector as the user would get a Ruby ArgumentError for misuse,
the spirit of this ticket also indicates that find with non-nil arg must be specified for non-insert bulk ops.
So this pull request checks for appropriate behavior and has tests to check improper usage of missing or nil query that should be specified via #find for non-insert bulk ops.
Also added a test for #find with trailing hash args usage and clarified some other tests relating to update arg usage.