spf: avoid wrong kind of json -> string conversion

When populating ehlo_domain in particular, we were using the json
Value::to_string() impl which produces a json-serialized string
which includes quotes.

This is a bit of a foot-gun, so let's add a get_meta_string method
to ConnectionMetaData to parallel the equivalent
Message::get_meta_string method.

crates/kumod/src/smtp_server.rs

@@ -2965,6 +2965,11 @@ impl ConnectionMetaData {
         meta.get(name.as_ref()).cloned()
     }
 
+    pub fn get_meta_string<N: AsRef<str>>(&self, name: N) -> Option<String> {
+        self.get_meta(name)
+            .and_then(|v| v.as_str().map(|s| s.to_string()))
+    }
+
     pub fn clone_inner(&self) -> serde_json::Value {
         self.map.lock().clone()
     }

crates/kumod/src/spf.rs

@@ -26,8 +26,8 @@ pub fn register<'lua>(lua: &'lua Lua) -> anyhow::Result<()> {
                     .and_then(|v| SocketAddr::from_str(v.as_str()?).ok())
                     .expect("`received_from` is always set, and always to a value representing a `SocketAddr`");
 
-                let ehlo_domain = meta.get_meta("ehlo_domain").map(|s| s.to_string());
-                let relaying_host_name = meta.get_meta("hostname").map(|s| s.to_string());
+                let ehlo_domain = meta.get_meta_string("ehlo_domain");
+                let relaying_host_name = meta.get_meta_string("hostname");
 
                 let resolver = dns_resolver::get_resolver();
                 let result = CheckHostParams {