convert string to int and lost '0' at beggin

[rikirill]

https://github.com/jpadilla/django-rest-framework-xml/blob/da33b6bd54e8e434a8218034f41678ec9d4a826e/rest_framework_xml/parsers.py#L74

for exm: if we will parsing 01234 we get dict {"test": 1234}

I propose add check on '0' at start of value