fix: preserve substitutions from To in Personalization

lib/mail/Personalization.php

@@ -43,11 +43,24 @@ class Personalization implements \JsonSerializable
 
     /**
      * Add a To object to a Personalization object
+     * If there are substitutions in the To object preserve them
+     * by transferring them to the Personalization object.
+     * If a substitution with the same key already exists
+     * that takes precedence, and we don't overwrite
      *
      * @param To $email To object
      */
     public function addTo($email)
     {
+        if ($subs = $email->getSubstitutions()) {
+            $existing_subs = $this->getSubstitutions();
+            foreach ($subs as $key => $value) {
+                if (!array_key_exists($key, $existing_subs)) {
+                    $this->addSubstitution($key, $value);
+                }
+            }
+        }
+
         $this->tos[] = $email;
     }
 

test/unit/PersonalizationTest.php

@@ -16,6 +16,7 @@
 use SendGrid\Mail\Subject;
 use SendGrid\Mail\To;
 use SendGrid\Mail\TypeException;
+use SendGrid\Mail\Substitution;
 
 /**
  * This class tests Personalization.
@@ -32,6 +33,32 @@ public function testAddTo()
         $this->assertSame('[email protected]', $personalization->getTos()[0]->getEmail());
     }
 
+    public function testAddToWithSubstitution()
+    {
+        $personalization = new Personalization();
+        $personalization->addTo(new To('[email protected]', 'Test with sub',
+            [
+                '-name-' => 'Example User'
+            ]
+        ));
+
+        $this->assertArrayHasKey('-name-', $personalization->getSubstitutions());
+    }
+
+    public function testAddSubstitutionHasPrecedence()
+    {
+        $personalization = new Personalization();
+        $personalization->addSubstitution(new Substitution('-name-', 'Example User 2'));
+        $personalization->addTo(new To('[email protected]', 'Test with sub',
+            [
+                '-name-' => 'Example User'
+            ]
+        ));
+
+        $this->assertArrayHasKey('-name-', $personalization->getSubstitutions());
+        $this->assertSame('Example User 2', $personalization->getSubstitutions()['-name-']);
+    }
+
     public function testAddFrom()
     {
         $personalization = new Personalization();