fix: Check error object before updating stack. (Fixes #700)

[janthoe]

Checking object returned by the assertion function in order to avoid property access errors when using assertion utilities like Chai, which would result in an error like this:

1) request(app)
       .expect(field, value[, fn])
         handling arbitrary expect functions
           doesn't create false negatives on assertion utilities:
     Error: Should not be called for non Error objects
      at Object.get (test/supertest.js:744:19)
      at ~/github/supertest/lib/test.js:29:367
      at Test._assertFunction (lib/test.js:99:170)
      at Test.assert (lib/test.js:66:173)
      at Server.localAssert (lib/test.js:54:664)
      at emitCloseNT (net.js:1657:8)
      at processTicksAndRejections (internal/process/task_queues.js:79:21)

[janthoe]

@niftylettuce PTAL

[niftylettuce]

v6.1.3 released to npm, any further issues please open another issue or submit a PR

thanks @janthoe https://github.com/visionmedia/supertest/releases/tag/v6.1.3

[coveralls]

Pull Request Test Coverage Report for Build 491

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.447%

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Totals
Change from base Build 489:	0.0%
Covered Lines:	146
Relevant Lines:	150

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💛 - Coveralls