SetFromTest: add new test for not overruling existing Sender

Based on the code in the method, any existing, previously set `Sender` should not be overruled, even when the `$auto` parameter is set to `true`.

This method tests that specific situation.

test/PHPMailer/SetFromTest.php

@@ -108,6 +108,20 @@ final class SetFromTest extends TestCase
         self::assertSame('', $this->Mail->Sender, 'Sender has been overruled');
     }
 
+    /**
+     * Test setting the From address, but not overruling an existing, non-empty Sender value.
+     */
+    public function testSetFromDoesNotOverruleExistingSender()
+    {
+        $sender             = 'donotoverrule@example.com';
+        $this->Mail->Sender = $sender;
+
+        $result = $this->Mail->setFrom('overruled@example.com');
+
+        self::assertTrue($result, 'setFrom failed');
+        self::assertSame($sender, $this->Mail->Sender, 'Sender has been overruled');
+    }
+
     /**
      * Test unsuccesfully setting the From, FromName and Sender properties.
      */