Allow entity_name to be called on parent classes

[urkle]

I am unsure why this limitation was done and issue #659 posses the same question.

This PR changes the behavior to allow calling entity_name if a parent class implements it.

My use case for this as follows

module Public::V2::Entities
   class Base < Grape::Entity
     # ... other shared logic
     def self.entity_name
       "V2::#{self.to_s.demodulize}"
     end
   end
   
   def MyEntity < Base
     # ....
   end
end

And I have ~ 20+ entities all inheriting from base and I don't really want the FULL module tree exposed in the API (in our application it is a little longer than what you see here). And being able to override entity_name once is significantly easier.

[coveralls]

Coverage remained the same at 99.469% when pulling b1f6d22 on urkle:feat-interited-entity-name into 961e7ca on ruby-grape:master.

[grape-bot]

	1 Error
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* [#794](https://github.com/ruby-grape/grape-swagger/pull/794): Allow entity_name to be inherited, fixes #659 - [@urkle](https://githubcom/urkle).

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[LeFnord]

thanks @urkle … it seems this is always WIP … 😉

[LeFnord]

@urkle … please can you have a look on it, want have it in in trhe next release

[urkle]

@LeFnord I re-based retested and corrected the rubocop issues

[LeFnord]

cool thanks @urkle