Handle non-ASCII-only in AS::MessagePack#signature?

Because `ActiveSupport::MessagePack::Serializer::SIGNATURE` includes a
non-ASCII-only byte (`"\xCC"`), it raises `Encoding::CompatibilityError`
when compared with another string that is not encoded with
`Encoding::BINARY` and also includes a non-ASCII-only byte.

To prevent that, this commit changes `AS::MessagePack#signature?` to
directly compare the first two bytes of both strings.

Fixes #48196.

activesupport/lib/active_support/message_pack/serializer.rb

@@ -25,7 +25,7 @@ def load(dumped)
       end
 
       def signature?(dumped)
-        dumped.start_with?(SIGNATURE)
+        dumped.getbyte(0) == SIGNATURE.getbyte(0) && dumped.getbyte(1) == SIGNATURE.getbyte(1)
       end
 
       def message_pack_factory

activesupport/test/message_pack/shared_serializer_tests.rb

@@ -41,6 +41,11 @@ module MessagePackSharedSerializerTests
       assert_not serializer.signature?("{}")
     end
 
+    test "#signature? handles non-ASCII-only non-binary-encoded strings" do
+      assert serializer.signature?(dump("ümlaut").force_encoding(Encoding::UTF_8))
+      assert_not serializer.signature?("ümlaut")
+    end
+
     test "roundtrips Symbol" do
       assert_roundtrip :some_symbol
     end