Add erc-sasl-auth-source-function to cached options
* lisp/erc/erc-sasl.el (erc-sasl--read-password): Consult cached
options instead of `erc-sasl-auth-source-function'.
(erc-sasl--init): Add `erc-sasl-auth-source-function' to
`erc-sasl--options'.
* test/lisp/erc/erc-sasl-tests.el (erc-sasl--read-password--basic,
erc-sasl--read-password--auth-source): Look for original value of
`erc-sasl-auth-source-function' in `erc-sasl--options' under the
`authfn' key.