improve error.showDiff behavior for multiline strings

[millermedeiros]

previously error.showDiff was conflicting with strings, so diff wasn't escaping invisible chars and also wasn't displaying line numbers.

now we make sure JSON.stringify() is only called if error.actual and/or error.expected aren't strings.

Before:

After:

[travisjeffery]

This should be fixed by 3cb9295

[rstacruz]

I think this needs to be revisited. Right now (v1.21.4 @ August 2014), the before output is what gets shown. This PR will actually work. 👍

[rstacruz]

To further elucidate, the issue here is that comparing two Strings will always get them JSON.stringified (actually utils.stringify now), which will always make the strings single-line. Using diff.diffWithWords will not help.

The lines in question look like this in v1.21.4:

    if (err.showDiff && sameType(actual, expected)) {
      escape = false;
      err.actual = actual = utils.stringify(actual);
      err.expected = expected = utils.stringify(expected);
    }

/* ... later ... */

function sameType(a, b) {
  a = Object.prototype.toString.call(a); /* returns something like "[object Object]" or "[object String]" */
  b = Object.prototype.toString.call(b);
  return a == b;
}

...I can't help but feel like the use of sameType isn't warranted here. This PR replaces that with a check that will only use .stringify if either the expected or actual objects are NOT strings, which IMHO makes more sense.

[rstacruz]

Digging a bit further, the use of sameType() here is linked to #900. the typeof actual != 'string' || typeof expected != 'string' will fix the same issue.