Fixed type issue. Create unit test for customer data model

Author: Andrew Chorniy

app/code/Magento/Customer/Model/Data/Customer.php

@@ -94,7 +94,7 @@ public function getCreatedAt()
      */
     public function getCreatedIn()
     {
-        return $this->_get(self::CREATED_IN);
+        return (string)$this->_get(self::CREATED_IN);
     }
 
     /**
@@ -150,7 +150,7 @@ public function getGender()
     /**
      * Get group id
      *
-     * @return string|null
+     * @return int|null
      */
     public function getGroupId()
     {

app/code/Magento/Customer/Test/Unit/Model/Data/CustomerTest.php

@@ -0,0 +1,74 @@
+<?php
+
+namespace Magento\Customer\Test\Unit\Model\Data;
+
+use Magento\Customer\Model\Data\Customer;
+use Magento\Framework\TestFramework\Unit\Helper\ObjectManager;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Unit test for customer data model
+ */
+class CustomerTest extends TestCase
+{
+    /** @var Customer */
+    protected $model;
+
+    /** @var ObjectManager */
+    protected $objectManager;
+
+    protected function setUp()
+    {
+        $this->objectManager = new ObjectManager($this);
+
+        $this->model = $this->objectManager->getObject(Customer::class);
+    }
+
+    /**
+     * Test getGroupId()
+     *
+     * @return void
+     */
+    public function testGetGroupId()
+    {
+        $testGroupId = 3;
+        $this->model->setGroupId($testGroupId);
+        $this->assertEquals($testGroupId, $this->model->getGroupId());
+    }
+
+    /**
+     * Test getCreatedIn()
+     *
+     * @param array|string $options
+     * @param array $expectedResult
+     *
+     * @dataProvider getCreatedInDataProvider
+     *
+     * @return void
+    */
+    public function testGetCreatedIn($options, $expectedResult)
+    {
+        for ($i = 0; $i < count($options); $i++) {
+            $this->model->setCreatedIn($options[$i]);
+            for ($j = $i; $j < count($expectedResult); $j++) {
+                $this->assertEquals($expectedResult[$j], $this->model->getCreatedIn());
+                break;
+            }
+        }
+    }
+
+    /**
+     * Data provider for testGetCreatedIn
+     *
+     * @return array
+     */
+    public function getCreatedInDataProvider()
+    {
+        return [
+            'array' => [
+                'options' => ['Default', 'Admin', 'US'],
+                'expectedResult' => ['Default', 'Admin', 'US']
+            ]
+        ];
+    }
+}