refactor(mimeparser): store only one signature fingerprint

Messages are normally not signed with more than one key
and in this case we pick an arbitrary signature later anyway.

src/mimeparser.rs

@@ -87,12 +87,12 @@ pub(crate) struct MimeMessage {
     pub chat_disposition_notification_to: Option<SingleInfo>,
     pub decrypting_failed: bool,
 
-    /// Set of valid signature fingerprints if a message is an
+    /// Valid signature fingerprint if a message is an
     /// Autocrypt encrypted and signed message.
     ///
     /// If a message is not encrypted or the signature is not valid,
-    /// this set is empty.
-    pub signatures: HashSet<Fingerprint>,
+    /// this is `None`.
+    pub signature: Option<Fingerprint>,
 
     /// The addresses for which there was a gossip header
     /// and their respective gossiped keys.
@@ -589,7 +589,7 @@ impl MimeMessage {
             decrypting_failed: mail.is_err(),
 
             // only non-empty if it was a valid autocrypt message
-            signatures,
+            signature: signatures.into_iter().last(),
             autocrypt_fingerprint,
             gossiped_keys,
             is_forwarded: false,
@@ -966,7 +966,7 @@ impl MimeMessage {
     /// This means the message was both encrypted and signed with a
     /// valid signature.
     pub fn was_encrypted(&self) -> bool {
-        !self.signatures.is_empty()
+        self.signature.is_some()
     }
 
     /// Returns whether the email contains a `chat-version` header.