add test for let with declared type (thx @amtoine)

crates/nu-command/tests/commands/let_.rs

@@ -23,6 +23,13 @@ fn let_takes_pipeline() {
     assert_eq!(actual.out, "11");
 }
 
+#[test]
+fn let_takes_pipeline_with_declared_type() {
+    let actual = nu!(r#"let x: list<string> = [] | append "hello world"; print $x.0"#);
+
+    assert_eq!(actual.out, "hello world");
+}
+
 #[test]
 fn let_pipeline_allows_in() {
     let actual =