Inputing a 2y salt should output a 2y hash

[dstufft]

Currently using bcrypt 3.0 with passlib is a bit broken, because it it inputs a precomputed $2y$ salt and expects to get back out a $2y$ hash (and it verifies that it does that), however instead it's getting back a $2b$ hash. Giving a $2b$ output when you give it a $2y$ input also means it breaks the most logical way to verify a bcrypt'd password since:

import bcrypt

assert bcrypt.hashpw(plaintext, hashed_with_2y) == hashed_with_2y

would fail because instead of a $2y$ we'd return a $2b$.

So this change will ensure that hashpw() returns the same prefix it was given originally. We still don't support the $2y$ prefix when passed into gensalt(), so getting a $2y$ out of hashpw() will still require either pre-existing data or manually constructing your own salt.

This also means our checkpw gets to be a little simpler since it no longer needs to deal with the fact that the output may differ only by the prefix.

[reaperhulk]

👍

[reaperhulk]

This needs a rebase

[dstufft]

Rebased!