Turn Option<AgentState> into AgentState (#149)

`Some(AgentState)` seem to be assumed pretty much everywhere. I could not
find any test or piece of code hinting at how `None` should be interpreted,
or even see how a state of `None` could even be constructed.

Co-authored-by: Ulrik <ulrikm@spotify.com>

src/request.rs

@@ -29,7 +29,7 @@ use super::SerdeValue;
 /// ```
 #[derive(Clone, Default)]
 pub struct Request {
-    pub(crate) agent: Arc<Mutex<Option<AgentState>>>,
+    pub(crate) agent: Arc<Mutex<AgentState>>,
 
     // via agent
     pub(crate) method: String,