json compares arguments against True/False by identity, not by boolean 
value; by example:

    if (skipkeys is False and ensure_ascii is True and
        check_circular is True and allow_nan is True ...

Using `ensure_ascii=1` won't work as intended. I don't see the reason 
to check those values by identity - they *are* boolean flags, and 
should be checked by value, as the usual practice.

The attached patch fixes the code and documentation (and a bug encoding 
True/False as keys, including unit tests)