[Fix rubocop#781] Fix a false positive for Rails/DynamicFindBy

Fixes rubocop#781.

This PR makes `Rails/DynamicFindBy` aware of `find_by_token_for`.
This default setting emphasizes suppressing false positive
over false negative.
`find_by_token_for` is probably rarely named, but it will
always exist in Rails in the future.

Author: koic

changelog/fix_a_false_positive_for_rails_dynamic_find_by.md

@@ -0,0 +1 @@
+* [#781](https://github.com/rubocop/rubocop-rails/issues/781): Make `Rails/DynamicFindBy` aware of `find_by_token_for`. ([@koic][])

config/default.yml

@@ -348,8 +348,10 @@ Rails/DynamicFindBy:
   # The `Whitelist` has been deprecated, Please use `AllowedMethods` instead.
   Whitelist:
     - find_by_sql
+    - find_by_token_for
   AllowedMethods:
     - find_by_sql
+    - find_by_token_for
   AllowedReceivers:
     - Gem::Specification
 

lib/rubocop/cop/rails/dynamic_find_by.rb

@@ -22,12 +22,14 @@ module Rails
       #   User.find_by(name: name, email: email)
       #   User.find_by!(email: email)
       #
-      # @example AllowedMethods: ['find_by_sql'] (default)
+      # @example AllowedMethods: ['find_by_sql', 'find_by_token_for'] (default)
       #   # bad
       #   User.find_by_query(users_query)
+      #   User.find_by_token_for(:password_reset, token)
       #
       #   # good
       #   User.find_by_sql(users_sql)
+      #   User.find_by_token_for(:password_reset, token)
       #
       # @example AllowedReceivers: ['Gem::Specification'] (default)
       #   # bad