[Fix rubocop#55] Fix an error for Minitest/AssertIncludes

Fixes rubocop#55

This PR fixes an error for `Minitest/AssertIncludes` when using local
variable argument.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#55](https://github.com/rubocop-hq/rubocop-minitest/issues/55): Fix an error for `Minitest/AssertIncludes` when using local variable argument. ([@koic][])
+
 ## 0.6.1 (2020-02-18)
 
 ### Bug fixes

lib/rubocop/cop/mixin/includes_cop_rule.rb

@@ -14,7 +14,7 @@ def rule(target_method:, prefer_method:)
           def on_send(node)
             return unless node.method?(:#{target_method})
             return unless (arguments = peel_redundant_parentheses_from(node.arguments))
-            return unless arguments.first.method?(:include?)
+            return unless arguments.first.respond_to?(:method?) && arguments.first.method?(:include?)
 
             add_offense(node, message: offense_message(arguments))
           end

test/rubocop/cop/minitest/assert_includes_test.rb

@@ -104,4 +104,15 @@ def test_do_something
       end
     RUBY
   end
+
+  def test_does_not_registers_offense_when_using_assert_with_local_variable
+    assert_no_offenses(<<~RUBY)
+      class FooTest < Minitest::Test
+        def test_do_something
+          collection = []
+          assert(collection)
+        end
+      end
+    RUBY
+  end
 end