Replaced <obj> instanceof Array with Array.isArray()

[iainbeeston]

I've been seeing a bug where at certain points in my tests, code to
retrieve elements like this:

driver.elementsByAccessibilityId("foo bar")

would throw an error:

[elementsByAccessibilityId("foo bar")] Response value field is not an Array.

After looking into the for wd code, I could see this was happening in
callbacks.js:133, in code that checked whether the return value was an
array, using instanceof Array. When I inspected what this code was
doing at runtime the value really was an array, but instanceof Array
was still returning false. Strangely I could reproduce the original
error when I ran my tests via Jest but not when I ran the same code in
the wd repl (where instanceof Array would return true at the same place).

It turns out that instanceof Array can return false for an array
created in another window or frame (or process?). See
http://web.mit.edu/jwalden/www/isArray.html for a full
explanation of this. Array.isArray() was added to ES5 for exactly this
reason, so I have replaced instanceof Array with Array.isArray()
throughout the codebase.

With this change I wd behaves as I would expect both in the repl and in
Jest.

[iainbeeston]

Thanks @sebv !