Dash's config validation prevents requests_pathname_prefix and requests_pathname_prefix from being set #364

Trying to supply values for either of requests_pathname_prefix and requests_pathname_prefix results in an error:

dash.exceptions.InvalidConfig:
    You supplied `url_base_pathname` and `requests_pathname_prefix`. This is ambiguous.
    To fix this, set `routes_pathname_prefix` instead of `url_base_pathname`.

However I did not supply a value for url_base_pathname. Looks like this is because _configs.pathname_configs is checking to see if url_base_pathname is not None to indicate that a user supplied their own value, but Dash sets a default value of '/', so this logic doesn't work.