ActiveRecord allows defining default scope as a class method, while Mongoid does not. To be as compatible with AR as possible, we should provide this option in Mongoid.

== Original Description ==
Nothing major just wondered why Mongoid works differently here.

At some Rails Performance Guide i read that scopes in general as well default_scope is just a macro that that is turning the given proc into a class method.

So to save a few nanoseconds of work and improve readability one should just write them as class methods.

This works for normal scopes but not for `default_scope`.

default_scope -> {
  where(:created_at.gte => 1.week.ago)
}
# this should do the same as it does with ActiveRecord
def self.default_scope
  where(:created_at.gte => 1.week.ago)
end