Fixed issue: Arr::get for ArrayObject on HHVM

HHVM 3.2.0 may emit a PHP Notice (Undefined Index) when calling
isset on an ArrayObject that maps the keys in some way - for example
the HTTP_Header object which converts all keys to lowercase - see
facebook/hhvm#3437

Thanks @acoulton
Merge pull request #754 from sergeyklay/feature-hhvm

Arr::get for ArrayObject on HHVM