Prefer encryption for the peer if the message is encrypted or signed with the known key (#3844)

Note that if the message is encrypted, we don't check whether it's signed with an attached key
currently, otherwise a massive refactoring of the code is needed because for encrypted messages a
signature is checked and discarded first now.

src/peerstate.rs

@@ -806,7 +806,6 @@ mod tests {
             verified_key_fingerprint: None,
             fingerprint_changed: false,
         };
-        assert_eq!(peerstate.prefer_encrypt, EncryptPreference::NoPreference);
 
         peerstate.apply_header(&header, 100);
         assert_eq!(peerstate.prefer_encrypt, EncryptPreference::Mutual);