[zendframework/zendframework#4996] Correct order of merging

- Added test showing issue with merging
- Fixed merging logic to merge new input with old

src/BaseInputFilter.php

@@ -73,9 +73,12 @@ public function add($input, $name = null)
         }
 
         if (isset($this->inputs[$name]) && $this->inputs[$name] instanceof InputInterface) {
-            // The element already exists, so merge the config. Please note that the order is important (already existing
+            // The element already exists, so merge the config. Please note
+            // that the order is important (already existing
             // input is merged with the parameter given)
-            $input->merge($this->inputs[$name]);
+            $original = $this->inputs[$name];
+            $original->merge($input);
+            return $this;
         }
 
         $this->inputs[$name] = $input;

test/BaseInputFilterTest.php

@@ -765,4 +765,22 @@ public function testGetInputs()
         $this->assertEquals('foo', $filters['foo']->getName());
         $this->assertEquals('bar', $filters['bar']->getName());
     }
+
+    /**
+     * @group 4996
+     */
+    public function testAddingExistingInputWillMergeIntoExisting()
+    {
+        $filter = new InputFilter();
+
+        $foo1    = new Input('foo');
+        $foo1->setRequired(true);
+        $filter->add($foo1);
+
+        $foo2    = new Input('foo');
+        $foo2->setRequired(false);
+        $filter->add($foo2);
+
+        $this->assertFalse($filter->get('foo')->isRequired());
+    }
 }