pkcs1v15: VerifyingKey::from to use prefixed version

As discussed in RustCrypto#341, `pkcs1v15::VerifyingKey::from` should use the
prefixed keys.

src/pkcs1v15/verifying_key.rs

@@ -173,7 +173,7 @@ where
     D: Digest,
 {
     fn from(key: RsaPublicKey) -> Self {
-        Self::new_unprefixed(key)
+        Self::new(key)
     }
 }