`find` should support _ids of type hash, array etc. 

The problem seems to be in Mongoid Array class extension - method "multi_arged?".
Although Hash instance is "resizable", we should treat it as a single "_id" when passed to "find" method, assuming that "_id" field is type of Hash.

Issue: [#2535]

Also author of the issue should bear in mind that Mongoid supports Hash key stored as String, not as Symbol instance [#2390]