Ruby 3.4 allows Strings as Fiber storage keys

But not anything else that responds to #to_sym. It does accept objects that
respond to #to_str.
This expands upon the ruby_bug block from upstream MRI. This also restores the
previously removed "can't use invalid keys" test, but with the String key
removed.

Reference: https://bugs.ruby-lang.org/issues/20978

core/fiber/storage_spec.rb

@@ -90,6 +90,13 @@
         key = :"#{self.class.name}#.#{self.object_id}"
         Fiber.new { Fiber[key] = 42; Fiber[key] }.resume.should == 42
       end
+
+      it "can't use invalid keys" do
+        invalid_keys = [Object.new, 12]
+        invalid_keys.each do |key|
+          -> { Fiber[key] }.should raise_error(TypeError)
+        end
+      end
     end
 
     ruby_bug "#20978", "3.2"..."3.4" do
@@ -98,6 +105,23 @@
         def key.to_str; "Foo"; end
         Fiber.new { Fiber[key] = 42; Fiber["Foo"] }.resume.should == 42
       end
+
+      it "converts a String key into a Symbol" do
+        Fiber.new { Fiber["key"] = 42; Fiber[:key] }.resume.should == 42
+        Fiber.new { Fiber[:key] = 42; Fiber["key"] }.resume.should == 42
+      end
+
+      it "can use any object that responds to #to_str as a key" do
+        key = mock("key")
+        key.should_receive(:to_str).twice.and_return("key")
+        Fiber.new { Fiber[key] = 42; Fiber[key] }.resume.should == 42
+      end
+    end
+
+    it "does not call #to_sym on the key" do
+      key = mock("key")
+      key.should_not_receive(:to_sym)
+      -> { Fiber[key] }.should raise_error(TypeError)
     end
 
     it "can access the storage of the parent fiber" do