Fix ClickTracking settings serialization

lib/helpers/mail/Mail.php

@@ -74,7 +74,10 @@ public function jsonSerialize()
             [
                 'enable' => $this->getEnable(),
                 'enable_text' => $this->getEnableText()
-            ]
+            ],
+            function ($value) {
+                return $value !== null;
+            }
         );
     }
 }

test/unit/ClickTrackingTest.php

@@ -0,0 +1,53 @@
+<?php
+
+/**
+ * ClickTracking Test. Tests JSON serialization of ClickTracking class.
+ *
+ * @author Aleksey Lavrinenko <[email protected]>
+ * @covers \SendGrid\ClickTracking
+ */
+class ClickTrackingTest extends \PHPUnit_Framework_TestCase
+{
+    public function testDefaultSerializesToEmptyArray()
+    {
+        $clickTracking = new \SendGrid\ClickTracking();
+
+        $this->assertEquals([], $clickTracking->jsonSerialize());
+    }
+
+    public function testSerializesTrueValues()
+    {
+        $clickTracking = new \SendGrid\ClickTracking();
+        $clickTracking->setEnable(true);
+        $clickTracking->setEnableText(true);
+
+        $this->assertEquals([
+            'enable' => true,
+            'enable_text' => true,
+        ], $clickTracking->jsonSerialize());
+    }
+
+    public function testSerializesFalseValues()
+    {
+        $clickTracking = new \SendGrid\ClickTracking();
+        $clickTracking->setEnable(false);
+        $clickTracking->setEnableText(false);
+
+        $this->assertEquals([
+            'enable' => false,
+            'enable_text' => false,
+        ], $clickTracking->jsonSerialize());
+    }
+
+    public function testSerializesVariousValues()
+    {
+        $clickTracking = new \SendGrid\ClickTracking();
+        $clickTracking->setEnable(false);
+        $clickTracking->setEnableText(true);
+
+        $this->assertEquals([
+            'enable' => false,
+            'enable_text' => true,
+        ], $clickTracking->jsonSerialize());
+    }
+}