[Singers] fixed openssl_pkcs7_encrypt() not using the encryptCipher

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #340
| License       | MIT

Swift_Signers_SMimeSigner did not use $encryptCipher, making it always use the the weakest cipher OPENSSL_CIPHER_RC2_40