Added OutputTest to verify issue #132 is a bug.

tests/methods/OutputTest.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace ParseCsv\tests\methods;
+
+use ParseCsv\Csv;
+use PHPUnit\Framework\TestCase;
+
+class OutputTest extends TestCase {
+
+    /**
+     * @runInSeparateProcess because download.php uses header()
+     */
+    public function testOutputWithFourParameters() {
+        $csv = new Csv();
+        $data = [0 => ['a', 'b', 'c'], 1 => ['d', 'e', 'f']];
+        $fields = ['col1', 'col2', 'col3'];
+        $output = $csv->output('test.csv', $data, $fields, ',');
+        $expected = "col1,col2,col3\ra,b,c\rd,e,f\r";
+        self::assertEquals($expected, $output);
+    }
+}