`pjos=0` bool value inverts the BIP78 spec

[spacebear21]

BIP78 states:

pjos=0: Signal to the sender that they MUST disallow payment output substitution. (See Unsecured payjoin server)

But our implementation does the inverse, and sets pjos to 1 when disabling output substitution!

            ("pjos", if self.disable_output_substitution { "1" } else { "0" }.to_string()),

This should be fixed to follow the spec (and tested properly).