[HttpFoundation] [Response] Precondition Failed

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.26

This feature adds the counterpart functions of `isNotModified()` and `setNotModified()`.
When a Request header ETag is provided and doesn't match the Response header Etag
AND/OR
When a Request header If-Modified-Since is provided and doesn't match the Response header Last-Modified

This allows to comply to RFC2616 specifications (see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.26).

This feature comes with the two following functions:
```php
hasPreconditionFailed()
setPreconditionFailed()
```

This might ease up conditional response management, especially when using this feature within a REST server.