Delegate Model#find with block argument to Enumerable#find

https://jira.mongodb.org/browse/MONGOID-4914 implemented delegation of association#find to Enumerable#find to filter the association in application process space.

The same can be done for top-level model classes.

Example code demonstrating the problem:

irb(main):005:0> Band.find { |x| p x }

=> nil

Here I expected the block to be called for each band and the first band to be returned, but I got nil and the block was not invoked.

See also the discussion in https://github.com/mongodb/mongoid/pull/4802.