[Fix #1488] Fix an error for Rails/ReadWriteAttribute cop

Resolves #1488

Basically we should not assume AST literals are not frozen.

Author: viralpraxis

changelog/fix_an_error_for_rails_read_write_attribute_cop.md

@@ -0,0 +1 @@
+* [#1488](https://github.com/rubocop/rubocop-rails/issues/1488): Fix an error for `Rails/ReadWriteAttribute` with a frozen string attribute name. ([@viralpraxis][])

lib/rubocop/cop/rails/read_write_attribute.rb

@@ -66,7 +66,7 @@ def within_shadowing_method?(node)
           return false unless enclosing_method
 
           shadowing_method_name = first_arg.value.to_s
-          shadowing_method_name << '=' if node.method?(:write_attribute)
+          shadowing_method_name += '=' if node.method?(:write_attribute)
           enclosing_method.method?(shadowing_method_name)
         end
 

spec/rubocop/cop/rails/read_write_attribute_spec.rb

@@ -235,6 +235,21 @@ def foo=(value)
       RUBY
     end
 
+    it 'corrects assignment with chained methods when using string attribute name' do
+      expect_offense(<<~RUBY)
+        def attr2=(k)
+          write_attribute("attr", 'test_' + postfix)
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `self["attr"] = 'test_' + postfix`.
+        end
+      RUBY
+
+      expect_correction(<<~RUBY)
+        def attr2=(k)
+          self["attr"] = 'test_' + postfix
+        end
+      RUBY
+    end
+
     it 'autocorrects multiline' do
       expect_offense(<<~RUBY)
         write_attribute(