Wrong handling of expressions with array index accessing object

[narcoticfresh]

Opening to get the Issue ID for the PR ;-)

Given the following test elements:

$targetDocument = '{"foo": {"bar": {"baz": [ {"bar":"baz"}, {"bar":"qux"} ] }}}';
$patchDocument = '[{"op":"replace", "path":"/foo/bar/baz/1/bar", "value":"otherValue"}]';
$expectedDocument = '{"foo": {"bar": {"baz": [ {"bar":"baz"}, {"bar":"otherValue"} ] }}}';

The test will fail. The problem is the current checks, where the expression /foo/bar/baz/1/bar will not traverse correctly because an overlooked is_array() check that needs changing..