generate_client_proof does not use leading zeros for salt parameter

[username223]

The function uses detail::encode_flip() which does not add padding.

From experimentation with the 1.12.1 client, this will result in incorrect proofs.

Changing

	const auto& salt_enc = detail::encode_flip(salt);

to

    const auto& salt_enc = detail::encode_flip_1363(salt, 32);

Seems to fix the problem.

There are other parameters in the function that also do not add padding, although I haven't gotten around to testing these yet.

[username223]

I accidentally used an old version of the development branch. This has already been fixed for salt as part of issue #27 .