Wrong attempts incrementation

Just a little PR, because some things looked a bit strange.

- Why increment two times the `$attempts` var, if the header is set ?
- Why nest the header `swarrot_retry_attempts` in a `header` key when resending the message ?

I also updated the tests that goes along with the rety test, to check that the `Message` sent has the same body and also the right value on its header.

I think that erasing the headers that way may be a bit harmful though... Why don't you simply overwrite the `swarrot_retry_attempts` ?