make sure builders / expectations API do not mutate the input struct

Not all APIs are pure right now:

- for example, the `tar` API mutates the builder object.
- some requests and header checks are also doing side effects (with the header map for example).

We want to make sure these do not mutate the input object so that a pattern like this is correct:

```go
req1 := SomeCheck().A().B()
req2 := req1.C().D()
```