Add start with zero cases to each unit test

tests/Feature/GenerateTest.php

@@ -17,21 +17,27 @@ public function generator()
 
         $this->assertEquals(2, (new Generator(9638527, Generator::GTIN_8))->getCheckDigit());
         $this->assertEquals(4, (new Generator(3216549, Generator::GTIN_8))->getCheckDigit());
-
+        $this->assertEquals(7, (new Generator("0012345", Generator::GTIN_8))->getCheckDigit());
+
         $this->assertEquals(2, (new Generator(91739456321, Generator::GTIN_12))->getCheckDigit());
         $this->assertEquals(8, (new Generator(74185245963, Generator::GTIN_12))->getCheckDigit());
+        $this->assertEquals(5, (new Generator("00123456789", Generator::GTIN_12))->getCheckDigit());
 
         $this->assertEquals(3, (new Generator(629104150021, Generator::GTIN_13))->getCheckDigit());
         $this->assertEquals(6, (new Generator(123456789876, Generator::GTIN_13))->getCheckDigit());
+        $this->assertEquals(5, (new Generator("001234567890", Generator::GTIN_13))->getCheckDigit());
 
         $this->assertEquals(6, (new Generator(7539514528423, Generator::GTIN_14))->getCheckDigit());
         $this->assertEquals(5, (new Generator(8563251459762, Generator::GTIN_14))->getCheckDigit());
+        $this->assertEquals(2, (new Generator("0012345678901", Generator::GTIN_14))->getCheckDigit());
 
         $this->assertEquals(0, (new Generator(7896541230123456, Generator::GSIN))->getCheckDigit());
         $this->assertEquals(3, (new Generator(7658485040650456, Generator::GSIN))->getCheckDigit());
+        $this->assertEquals(3, (new Generator("0012345678901234", Generator::GSIN))->getCheckDigit());
 
         $this->assertEquals(6, (new Generator(95135623050123698, Generator::SSCC))->getCheckDigit());
         $this->assertEquals(7, (new Generator(87643802105978513, Generator::SSCC))->getCheckDigit());
+        $this->assertEquals(2, (new Generator("00123456789012345", Generator::SSCC))->getCheckDigit());
     }
 
     /**