fix empty Header can't iterate

Author: lihsai0

src/Qiniu/Http/Header.php

@@ -8,7 +8,7 @@
 class Header implements \ArrayAccess, \IteratorAggregate, \Countable
 {
     /** @var array normalized key name map */
-    private $data;
+    private $data = array();
 
     /**
      * @param array $obj non-normalized header object

tests/Qiniu/Tests/HeaderTest.php

@@ -145,6 +145,21 @@ public function testGetIterator()
         $this->assertFalse($hasException);
     }
 
+    public function testEmptyHeaderIterator()
+    {
+        $emptyHeader = new Header();
+
+        $hasException = false;
+        try {
+            foreach ($emptyHeader as $k => $v) {
+                $hasException = !isset($header[$k]);
+            }
+        } catch (\Exception $e) {
+            $hasException = true;
+        }
+        $this->assertFalse($hasException);
+    }
+
     public function testCount()
     {
         $header = new Header($this->heads);