Do not accept prefer-encrypt=reset value from emails

Reset is an internal value that received messages should not be able to set.

Also return an error on any value other than "mutual" and "nopreference", errors are converted to NoPreference by the caller.

src/aheader.rs

@@ -44,8 +44,8 @@ impl str::FromStr for EncryptPreference {
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match s {
             "mutual" => Ok(EncryptPreference::Mutual),
-            "reset" => Ok(EncryptPreference::Reset),
-            _ => Ok(EncryptPreference::NoPreference),
+            "nopreference" => Ok(EncryptPreference::NoPreference),
+            _ => Err(()),
         }
     }
 }