to avoid accidentally mutating the original's state when doing:

```ruby
req2 = req.dup
req2.concat([">= 3.3.22"])
```

see https://github.com/rake-compiler/rake-compiler/pull/236 for a
real-world use case that would be made simpler with this behavior.

https://github.com/rubygems/rubygems/commit/8e0c03144e