Fix for lnkedin/dustjs issue #210. All tests passed.

This is a fix for issue #210. All build tests pass and it runs on the browser too.

This fixes a problem on older browsers which do not have Array.isArray(), like IE8 and older.

Since "compiler.js" apparently can be used independently, simply replacing its "Array.isArray" call by a "dust.isArray" call breaks the tests. Therefore I also repeat the initialization of the dust.isArray property with the appropriate method when this one is not yet filled at compiler creation.