htmlentities() doesn't recognize (and ignores) ENT_HTML5 flag

``` php
var_dump(htmlentities('This + is. a < test', ENT_HTML5 ));
```

on `HipHop VM 3.1.0-dev+2014.05.22 (rel)` produces:

```
string(22) "This + is. a < test
```

on `PHP 5.5.12-2` produces:

```
string(34) "This + is. a < test"
```