Fix '0' being replaces by first string's value

Fixes #99

src/Minify.php

@@ -318,7 +318,7 @@ abstract class Minify
         $minifier = $this;
         $callback = function ($match) use ($minifier) {
             // check the second index here, because the first always contains a quote
-            if (!$match[2]) {
+            if ($match[2] === '') {
                 /*
                  * Empty strings need no placeholder; they can't be confused for
                  * anything else anyway.

tests/js/JSTest.php

@@ -679,6 +679,12 @@ BUG
             'if(!0){if(!0)console.log("test")}',
         );
 
+        // https://github.com/matthiasmullie/minify/issues/99
+        $tests[] = array(
+            '"object";"object2";"0";"1"',
+            '"object";"object2";"0";"1"',
+        );
+
         //update tests' expected results for cross-system compatibility
         foreach ($tests as &$test) {
             if (!empty($test[1])) {