Add withLength() immutable helper

Add the immutable variant of length().

src/Network/Response.php

@@ -1541,6 +1541,7 @@ public function protocol($protocol = null)
      *
      * @param int|null $bytes Number of bytes
      * @return int|null
+     * @deprecated 3.4.0 Use withLength() to set length instead.
      */
     public function length($bytes = null)
     {
@@ -1555,6 +1556,17 @@ public function length($bytes = null)
         return null;
     }
 
+    /**
+     * Create a new response with the Content-Length header set.
+     *
+     * @param int|string $bytes Number of bytes
+     * @return static
+     */
+    public function withLength($bytes)
+    {
+        return $this->withHeader('Content-Length', (string)$bytes);
+    }
+
     /**
      * Checks whether a response has not been modified according to the 'If-None-Match'
      * (Etags) and 'If-Modified-Since' (last modification date) request

tests/TestCase/Network/ResponseTest.php

@@ -695,6 +695,23 @@ public function testLength()
         $this->assertEquals('100', $response->getHeaderLine('Content-Length'));
     }
 
+    /**
+     * Tests settings the content length
+     *
+     * @return void
+     */
+    public function testWithLength()
+    {
+        $response = new Response();
+        $this->assertFalse($response->hasHeader('Content-Length'));
+
+        $new = $response->withLength(100);
+        $this->assertFalse($response->hasHeader('Content-Length'), 'Old instance not modified');
+
+        $this->assertSame('100', $new->getHeaderLine('Content-Length'));
+        $this->assertSame('100', $new->length(), 'new method is compat with old.');
+    }
+
     /**
      * Tests setting the expiration date
      *