test: buffer should always be stringified

This test makes sure that independently of the buffer type, the input
is always stringified and generates a valid input.

test/parallel/test-fs-buffertype-writesync.js

@@ -0,0 +1,20 @@
+'use strict';
+const common = require('../common');
+
+// This test ensures that writeSync does support inputs which
+// are then correctly converted into string buffers.
+
+const assert = require('assert');
+const fs = require('fs');
+const path = require('path');
+
+const filePath = path.join(common.tmpDir, 'test_buffer_type');
+const v = [true, false, 0, 1, Infinity, common.noop, {}, [], undefined, null];
+
+common.refreshTmpDir();
+
+v.forEach((value) => {
+  const fd = fs.openSync(filePath, 'w');
+  fs.writeSync(fd, value);
+  assert.strictEqual(fs.readFileSync(filePath).toString(), value + '');
+});