return hasher when value is null

[browner12]

this allows us to use the other methods on the Hasher (check, needsRehash).

similar to many of the other helper function that simply return the object if no value is passed.

[Dylan-DPC-zz]

Isn't this a breaking change?

[browner12]

@Dylan-DPC and I discussed this in Slack. Found out that bcrypt(null) actually returns a hashed string, so this is technically a break.

Will close and retarget to master.

Thanks!