Lift single-branch $or conditions to top level

When $or produces a single branch, the query can be simplified to contain just the branch on the top level. For example:

irb(main):008:0> Band.or(hello:1)

=> 

#<Mongoid::Criteria

  selector: {"$or"=>[{"hello"=>1}]}

  options:  {}

  class:    Band

  embedded: false>

# equivalent to

 => 

#<Mongoid::Criteria

  selector: {"hello"=>1}

  options:  {}

  class:    Band

  embedded: false>

Note that, per https://www.mongodb.com/docs/mongoid/master/reference/queries/#logical-operations, this simplification must only be performed when the left side of .or doesn't have any conditions and right side has only one condition, i.e. it is insufficient to just check the arguments to .or.