Fix the error that is wrong link title of a downloadable product when enabling "Use Default Value"

Author: tna274

app/code/Magento/Downloadable/Model/Link/Builder.php

@@ -116,7 +116,7 @@ public function build(\Magento\Downloadable\Api\Data\LinkInterface $link)
             $link->setLinkFile($linkFileName);
             $link->setLinkUrl(null);
         }
-        
+
         if (isset($this->data['sample'])) {
             $link = $this->buildSample($link, $this->data['sample']);
         }
@@ -132,6 +132,10 @@ public function build(\Magento\Downloadable\Api\Data\LinkInterface $link)
         if (isset($this->data['is_unlimited']) && $this->data['is_unlimited']) {
             $link->setNumberOfDownloads(0);
         }
+
+        if (isset($this->data['use_default_title']) && $this->data['use_default_title'] == '1') {
+            $link->setTitle(null);
+        }
         $this->resetData();
 
         return $link;

app/code/Magento/Downloadable/Test/Unit/Model/Link/BuilderTest.php

@@ -68,7 +68,7 @@ protected function setUp()
         $this->linkMock = $this->getMockBuilder(LinkInterface::class)
             ->disableOriginalConstructor()
             ->getMockForAbstractClass();
-        
+
         $this->service = $objectManagerHelper->getObject(
             Builder::class,
             [
@@ -160,6 +160,9 @@ public function testBuild($data, $expectedPrice)
         if (isset($data['is_unlimited'])) {
             $this->linkMock->expects($this->once())->method('setNumberOfDownloads')->with(0);
         }
+        if (isset($data['use_default_title']) && $data['use_default_title'] == '1') {
+            $this->linkMock->expects($this->once())->method('getTitle')->with(null);
+        }
         if (isset($data['price'])) {
             $this->linkMock->expects($this->once())->method('getPrice')->willReturn($data['price']);
         } else {