Make str collect take an optional separator value (#2289)

* Make `str collect` take an optional separator value

* Make `str collect` take an optional separator value

* Add some tests

* Fix my tests

crates/nu-cli/tests/commands/str_/collect.rs

@@ -0,0 +1,53 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn test_1() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo 1..5 | str from | str collect 
+        "#
+        )
+    );
+
+    assert_eq!(actual.out, "12345");
+}
+
+#[test]
+fn test_2() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo [a b c d] | str collect "<sep>"
+        "#
+        )
+    );
+
+    assert_eq!(actual.out, "a<sep>b<sep>c<sep>d");
+}
+
+#[test]
+fn construct_a_path() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo [sample txt] | str collect "."
+        "#
+        )
+    );
+
+    assert_eq!(actual.out, "sample.txt");
+}
+
+#[test]
+fn sum_one_to_four() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo 1..4 | str from | str collect "+" | math eval
+        "#
+        )
+    );
+
+    assert!(actual.out.contains("10.0"));
+}

crates/nu-cli/tests/commands/str_/mod.rs

@@ -1,3 +1,5 @@
+mod collect;
+
 use nu_test_support::fs::Stub::FileWithContent;
 use nu_test_support::playground::Playground;
 use nu_test_support::{nu, pipeline};