Speedup non-changed images check

[k-g-a]

Before calling actual diffImage we can check for two Buffer's (received and expected) equality. If those are equal, we can skip actual images comparison.

[coveralls]

Pull Request Test Coverage Report for Build 48

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 97.148%

---

Totals
Change from base Build 46:	0.03%
Covered Lines:	391
Relevant Lines:	391

---

💛 - Coveralls