Use more robust Array type check

[rubendg]

Improved robustness of Array type check by using isArray instead of instanceof.

I have a project with supertest (^3.1.0) and Jest (^23.0.0) as test runner. For some reason, that I haven't yet been able to pinpoint, the array instance check fails for me.

Vars in scope:

actual = ["a"]
field = "set-cookie"
fieldExpected = "a"

when I evaluate the following expressions I get:

actual instanceof Array -> false
Array.isArray(actual) -> true

changing the check to use isArray makes it work for me.

I suspect I'm running into some of the issues described over here: http://perfectionkills.com/instanceof-considered-harmful-or-how-to-write-a-robust-isarray/

[amoshaviv]

👍

[coveralls]

Pull Request Test Coverage Report for Build 335

• 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 97.285%

---

Totals
Change from base Build 330:	0.0%
Covered Lines:	136
Relevant Lines:	140

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 335

• 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 97.285%

---

Totals
Change from base Build 330:	0.0%
Covered Lines:	136
Relevant Lines:	140

---

💛 - Coveralls

[rimiti]

Thank you @rubendg for your fix.
I'll merge it in the next release. 👍