Allow block form in Mongoid::Association::EmbedsMany::Proxy#count

Mongoid::Association::EmbedsMany::Proxy#count should allow block form to make it congruent with #any?, #none?, #all?, etc.

Example:
Person.addresses.count

I've also added additional tests for other methods #any?, #none?, #all?
 

https://github.com/mongodb/mongoid/pull/5000