Add one test instance

.travis.yml

@@ -13,7 +13,7 @@ env:
   matrix:
     - GITBOOK_VERSION=2.6.7
     - GITBOOK_VERSION=3.2.2
-before_script:
+install:
   - npm install
   - npm install gitbook@$GITBOOK_VERSION
 script:

package.json

@@ -49,7 +49,8 @@
     "gulp": "^3.9.1",
     "istanbul": "^0.4.5",
     "mocha": "^3.1.0",
-    "sinon": "^1.17.6"
+    "sinon": "^1.17.6",
+    "sinon-chai": "^2.8.0"
   },
   "peerDependencies": {
     "gitbook": ">=2.5.0"

test/helper.js

@@ -0,0 +1,10 @@
+'use strict';
+
+var chai = require('chai');
+
+var sinonChai = require('sinon-chai');
+chai.use(sinonChai);
+
+module.exports = {
+  expect: chai.expect
+};

test/isNullable.test.js

@@ -1,5 +1,20 @@
 'use strict';
 
+var expect = require('./helper').expect;
+
+var gulpGitbook = require('..');
+var isNullable = gulpGitbook._private.isNullable;
+
 describe('isNullable', function () {
 
+  it('should be ok for nullable values', function () {
+    expect(isNullable(null)).to.be.true;
+    expect(isNullable(undefined)).to.be.true;
+    expect(isNullable(false)).to.be.false;
+    expect(isNullable(0)).to.be.false;
+    expect(isNullable('')).to.be.false;
+    expect(isNullable({})).to.be.false;
+    expect(isNullable(true)).to.be.false;
+  });
+
 });