Return 405 Method Not Allowed for unsupported method for route (#63)

Fixes https://github.com/tokio-rs/axum/issues/61

src/service/mod.rs

@@ -256,7 +256,7 @@ where
             inner: svc,
             _request_body: PhantomData,
         },
-        fallback: EmptyRouter::new(),
+        fallback: EmptyRouter::method_not_allowed(),
     }
 }