WIP: 1486/first row as headers (#1530)

* headers plugin

* Remove plugin

* Add non-functioning headers command

* Add ability to extract headers from first row

* Refactor header extraction

* Rebuild indexmap with proper headers

* Rebuild result properly

* Compiling, probably wrapped too much?

* Refactoring

* Deal with case of empty header cell

* Deal with case of empty header cell

* Fix formatting

* Fix linting, attempt 2.

* Move whole_stream_command(Headers) to more appropriate section

* ... more linting

* Return Err(ShellError...) instead of panic, yield each row instead of entire table

* Insert Column[index] if no header info is found.

* Update error description

* Add initial test

* Add tests for headers command

* Lint test cases in headers

* Change ShellError for headers, Add sample_headers file to utils.rs

* Add empty sheet to test file

* Revert "Add empty sheet to test file"

This reverts commit a4bf38a31d.

* Show error message when given empty table

crates/nu-cli/tests/commands/headers.rs

@@ -0,0 +1,31 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn headers_uses_first_row_as_header() {
+    let actual = nu!(
+    cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            open sample_headers.xlsx
+            | get Sheet1
+            | headers
+            | get header0
+            | from-json"#
+    ));
+
+    assert_eq!(actual, "r1c0r2c0")
+}
+
+#[test]
+fn headers_adds_missing_column_name() {
+    let actual = nu!(
+    cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            open sample_headers.xlsx
+            | get Sheet1
+            | headers
+            | get Column1
+            | from-json"#
+    ));
+
+    assert_eq!(actual, "r1c1r2c1")
+}

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

@@ -10,6 +10,7 @@ mod first;
 mod format;
 mod get;
 mod group_by;
+mod headers;
 mod histogram;
 mod insert;
 mod last;