Proposal: BodyTextAssertion should also contain the actual response body when failing

[chrisbbe]

Hi,

Thanks for the excellent framework. I'm really keen to enhance the error message in 'BodyTextAssertion' and include the actual response payload.

I´ll provide a PR.

[Hawxy]

All assertions should already include the body per https://github.com/JasperFx/alba/blob/master/src/Alba/ScenarioAssertionException.cs#L57