Having equivalent ensure! methods

[cbonaudo]

Using assert_body_matches, and expect_status are used to panic during the tests.

I think that having an equivalent with ensure!-like functionnality could be a nice possibility if that's what we want to use in our tests.

• ensure_body_matches, that returns a Result if body do not match
• ensure_status, that returns a Result if the status do not match

!!! - It could make sense to split the expect_status and the json() of the response before implementing this so "ensure_status" doesn't need to json() the response too.