Describe the bug

According to docstrings for the convert_date method, illegal values should be returned as None. In fact, the method returns the original date value as string

To Reproduce

Code:

import pymysql.converters

pymysql.converters.convert_date('2007-02-31')
pymysql.converters.convert_date('0000-00-00')

Returned values are '2007-02-31' and '0000-00-00'

Expected behavior

According to the docstring, expected return values would be None:

      >>> convert_date('2007-02-31') is None
      True
      >>> convert_date('0000-00-00') is None
      True

Environment

• OS: Linux
• PyMySQL version: 1.0.2