test: improve test-stream2-objects.js

This commit improves the test cases in
test-stream2-objects.js by using assert.strictEqual
instead of assert.equal.

This is a part of Code And Learn at NodeFest 2016

Fixes: https://github.com/nodejs/code-and-learn/issues/58
PR-URL: https://github.com/nodejs/node/pull/9565


Reviewed-By: Shigeki Ohtsu <ohtsu@ohtsu.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>