Deserializing invalid characters in a base64 string does not raise an error

[SrirachaHorse]

If deserializing a base16 string with invalid characters, binascii.unhexlify() throws a validation error. This is caught by xsdata and thrown as a ConverterError.

By comparison if deserializing a base64 string with invalid characters, b64decode() does not throw a validation error, as no validate flag is being passed to the decode method. Instead, the method will ignore any invalid characters, which is less than desirable.

If base16 strings are throwing validation errors on invalid characters then base64 strings should be treated similarly.