bug #22036 Set Date header in Response constructor already (mpdude)

This PR was squashed before being merged into the 2.8 branch (closes #22036).

Discussion
----------

Set Date header in Response constructor already

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Setting the `Date` header in the `Response` constructor has been removed in #14912 and changed to a more lazy approach in `getDate()`.

That way, methods like `getAge()`, `getTtl()` or `isFresh()` cause side effects as they eventually call `getDate()` and the Request "starts to age" once you call them.

I don't know if this would be a nice test, but current behaviour is

```php
        $response = new Response();
        $response->setSharedMaxAge(10);
        sleep(20);
        $this->assertTrue($response->isFresh());
        sleep(5);
        $this->assertTrue($response->isFresh());
        sleep(5);
        $this->assertFalse($response->isFresh());
```

A particular weird case is the `isCacheable()` method, because it calls `isFresh()` only under certain conditions, like particular status codes, no `ETag` present etc. This symptom is also described under "Cause of the problem" in #19390, however the problem is worked around there in other ways.

So, this PR suggests to effectively revert #14912.

Additionally, I'd like to suggest to move this special handling of the `Date` header into the `ResponseHeaderBag`. If the `ResponseHeaderBag` guards that we always have the `Date`, we would not need special logic in `sendHeaders()` and could also take care of #14912 (comment).

Commits
-------

3a7fa7e Set Date header in Response constructor already

src/Symfony/Component/HttpFoundation/Response.php

@@ -201,6 +201,11 @@ public function __construct($content = '', $status = 200, $headers = array())
         $this->setContent($content);
         $this->setStatusCode($status);
         $this->setProtocolVersion('1.0');
+
+        /* RFC2616 - 14.18 says all Responses need to have a Date */
+        if (!$this->headers->has('Date')) {
+            $this->setDate(new \DateTime(null, new \DateTimeZone('UTC')));
+        }
     }
 
     /**
@@ -329,6 +334,7 @@ public function sendHeaders()
             return $this;
         }
 
+        /* RFC2616 - 14.18 says all Responses need to have a Date */
         if (!$this->headers->has('Date')) {
             $this->setDate(\DateTime::createFromFormat('U', time()));
         }
@@ -612,6 +618,11 @@ public function mustRevalidate()
      */
     public function getDate()
     {
+        /*
+            RFC2616 - 14.18 says all Responses need to have a Date.
+            Make sure we provide one even if it the header
+            has been removed in the meantime.
+         */
         if (!$this->headers->has('Date')) {
             $this->setDate(\DateTime::createFromFormat('U', time()));
         }

src/Symfony/Component/HttpFoundation/Tests/ResponseTest.php

@@ -276,8 +276,10 @@ public function testGetDate()
         $this->assertEquals($now->getTimestamp(), $date->getTimestamp(), '->getDate() returns the date when the header has been modified');
 
         $response = new Response('', 200);
+        $now = $this->createDateTimeNow();
         $response->headers->remove('Date');
-        $this->assertInstanceOf('\DateTime', $response->getDate());
+        $date = $response->getDate();
+        $this->assertEquals($now->getTimestamp(), $date->getTimestamp(), '->getDate() returns the current Date when the header has previously been removed');
     }
 
     public function testGetMaxAge()