Selector.multi_selection? returns incorrectly for the symbol equivalents of "$and" and "$nor" (i.e., :$and and :$nor). This incorrect behavior breaks Selector.merge!(), which is a public Mongoid API exposed to users.

The PR is already ready to be reviewed/merged: https://github.com/mongodb/mongoid/pull/4981

The PR:

• Fixes this issue.
• Provides a unit test that repros the problem in the old code, and now passes with the new code.
• Provides a user-level reproduction of the broken behavior using public Mongoid APIs.