add merge tune to extract tunes by title

update Tune::merge() and add Tune::equals()

tests/unit/AbcParser/Application/UseCases/ExtractTuneFromCollectionTest.php

@@ -35,7 +35,8 @@ class ExtractTuneFromCollectionTest extends \Codeception\Test\Unit
 
         $result = $extractor->extractTuneByTitle($title);
 
-        $this->assertCount(1, $result);
-        $this->assertNotEquals($tunes, $result);
+        foreach ($tunes as $tune) {
+            $this->assertNotEquals($tune, $result);
+        }
     }
 }