use 'given' to wrap multiple assertions

[katiedotson]

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Fix for #421 uses given to wrap multiple assertions. One potential issue here is if there are multiple failures in a block, only the first will be shown as a cause. I might suggest making a separate function to solve for this.

[rybick]

I have created a different fix for https://github.com/willowtreeapps/assertk/issues/421 a couple of days ago: https://github.com/willowtreeapps/assertk/pull/422 It doesn't have the problem that it shows only first error in a block, but is a little more complex.

[evant]

Should be fixed by https://github.com/willowtreeapps/assertk/pull/437