Fix Arr::get for ArrayObject on HHVM - fixes HTTP_Header behaviour

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
https://github.com/facebook/hhvm/issues/3437

Implemented a workaround in Arr::get to resolve breaking tests for
Kohana_Request_Client on HHVM.