[Fix #51] Add ability to whitelist receiver class names

[tejasbubane]

In Rails/DynamicFindBy.

Fixes #51.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[krzysiek1507]

Maybe we can check if the receiver inherits from ActiveRecord::Base?

[tejasbubane]

Maybe we can check if the receiver inherits from ActiveRecord::Base?

This is not reliable. A class can extend from ApplicationRecord instead - or even from other classes.

[krzysiek1507]

A = Class.new ActiveRecord::Base 
B = Class.new A

B <= ActiveRecord::Base
=> true

[andyw8]

@krzysiek1507 RuboCop uses static analysis, It doesn't run the code, so this approach isn't possible.

[ngouy]

Thank you for this PR :)

[tejasbubane]

Rebased & fixed merge conflicts. @koic

[tejasbubane]

@koic Fixed as per review comments.

[tejasbubane]

@koic Thanks for the review :) I have fixed as per your suggestions.

[koic]

Nicely done 👍