Add trillium::Conn::response_body() to borrow the response body

trillium_http::Conn supports returning a borrow of the response body,
but trillium:Conn doesn't have a corresponding method.

This is useful for examining a response body non-destructively without
having to take it and re-set it afterwards.

trillium/src/conn.rs

@@ -204,6 +204,24 @@ impl Conn {
         self.inner.take_response_body()
     }
 
+    /**
+    Borrows the response body from the `Conn`
+
+    ```
+    use trillium_testing::prelude::*;
+    let mut conn = get("/").on(&());
+
+    conn.set_body("hello");
+    let body = conn.response_body().unwrap();
+    assert_eq!(body.len(), Some(5));
+    assert!(body.is_static());
+    assert_eq!(body.static_bytes(), Some(&b"hello"[..]));
+    ```
+    */
+    pub fn response_body(&self) -> Option<&Body> {
+        self.inner.response_body()
+    }
+
     /**
     Attempts to retrieve a &T from the state set