[WIP] [HttpFoundation] Fixed JsonResponse for IE 7-10

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10693
| License       | MIT

Related to issue #10693: Internet Explorer doesn't support 'application/json' (it offers to download a document instead of printing the object).
This seems to affect the [versions 7 to 10](http://stackoverflow.com/questions/13943439/json-response-download-in-ie710). 

To fix this, IE has to be detected (beware of Opera which uses a similar user-agent) and a 'text/json' content type should be set.

To do:

* [x] Write a test
* [ ] Actually test it on IE
I was curious about what alternative we could come up with. Should `JsonResponse` depend on `Request`? Should we introduce a new `UserAgent` class?