Fix #13: Remove pass by reference usage; Add "Array to string conversion" test

Curl.class.php

@@ -135,9 +135,9 @@ class Curl {
                 // Fix "Notice: Array to string conversion" when $value in
                 // curl_setopt($ch, CURLOPT_POSTFIELDS, $value) is an array
                 // that contains an empty array.
-                foreach ($data as &$value) {
+                foreach ($data as $key => $value) {
                     if (is_array($value) && empty($value)) {
-                        $value = '';
+                        $data[$key] = '';
                     }
                 }
             }

tests/run.php

@@ -241,4 +241,14 @@ class CurlTest extends PHPUnit_Framework_TestCase {
             $test->curl->request_headers);
         $this->assertTrue(!empty($content_type));
     }
+
+    public function testArrayToStringConversion() {
+        $test = new Test();
+        $test->server('post', 'POST', array(
+            'foo' => 'bar',
+            'baz' => array(
+            ),
+        ));
+        $this->assertTrue($test->curl->response === 'foo=bar&baz=');
+    }
 }