Ensure the assertion message is a string

[GMartigny]

This will check for the presence of the extra argument (which should be the message) and verify if it's a string.
It will try to lookup variable and its value in the scope, but non-linear flow could break it.

let message;
function woops () {
	message = null;
}
message = 'ok';
woops();
t.is('same', 'same', message); // <= this will not raise an error

Should we cover this ?

Fixes #167

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #167: The assertion-arguments rule should validate message argument type

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[sindresorhus]

Should we cover this ?

No. Too hard to get right and too much of an edge-case.