[Fix rubocop#209] Mark Minitest/AssertTruthy as unsafe autocorrection

Fixes rubocop#209.

This PR marks `Minitest/AssertTruthy` as unsafe autocorrection.

changelog/change_mark_minitest_assert_truthy_as_unsafe_autocorrection.md

@@ -0,0 +1 @@
+* [#209](https://github.com/rubocop/rubocop-minitest/issues/209): Mark `Minitest/AssertTruthy` as unsafe autocorrection. ([@koic][])

config/default.yml

@@ -103,7 +103,9 @@ Minitest/AssertTruthy:
   Description: 'This cop enforces the test to use `assert(actual)` instead of using `assert_equal(true, actual)`.'
   StyleGuide: 'https://minitest.rubystyle.guide#assert-truthy'
   Enabled: true
+  SafeAutoCorrect: false
   VersionAdded: '0.2'
+  VersionChanged: '<<next>>'
 
 Minitest/AssertWithExpectedArgument:
   Description: 'This cop tries to detect when a user accidentally used `assert` when they meant to use `assert_equal`.'

lib/rubocop/cop/minitest/assert_truthy.rb

@@ -5,6 +5,9 @@ module Cop
     module Minitest
       # Enforces the test to use `assert(actual)` instead of using `assert_equal(true, actual)`.
       #
+      # @safety
+      #   This cop's autocorrection is unsafe because true might be expected instead of truthy.
+      #
       # @example
       #   # bad
       #   assert_equal(true, actual)