use Array.isArray if available for mixed-context array's, fallback to instanceof

When data comes in from a different context, data instanceof Array will return false, and render indexed keys as a xml node. (see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof)

Array.isArray isn't affected by this.
Merge pull request #17 from mangrove/master

Mixed-Context array checking fails, use Array.isArray