Removes two invalid tests

`MessageTraitTest` had two tests that were for behavior not supported by
PSR-7:

- `testHeaderExistsIfWithNoValues`
- `testHeaderWithNoValues`

As the integration tests demonstrate, these were not valid scenarios. As
such, the tests are removed.

Author: weierophinney

composer.lock

@@ -135,22 +135,6 @@ public function testAddHeaderAppendsToExistingHeader()
         $this->assertSame('Foo,Bar', $message2->getHeaderLine('X-Foo'));
     }
 
-    public function testHeaderExistsIfWithNoValues()
-    {
-        $message = $this->message->withHeader('X-Foo', []);
-
-        $this->assertTrue($message->hasHeader('X-Foo'));
-    }
-
-    public function testHeaderWithNoValues()
-    {
-        $message = $this->message->withHeader('X-Foo', []);
-
-        $this->assertSame([], $message->getHeader('X-Foo'));
-        $this->assertSame('', $message->getHeaderLine('X-Foo'));
-    }
-
-
     public function testCanRemoveHeaders()
     {
         $message = $this->message->withHeader('X-Foo', 'Foo');