Allow UTF-8 characters in overlay_args

[martinzamuner]

Using UTF-8 characters inside overlay_args generates different problems.

Case #1

link_to_overlay "Test", test_path, overlay_args: { title: "Test thís" }

raises an ArgumentError with invalid byte sequence in UTF-8.

Case #2

link_to_overlay "Test", test_path, overlay_args: { title: "✏️ Test" }

will simply behave as a non-overlay request.