Support JSON response in Internet Explorer < 10

Internet Explorer older than 10 does not accept
Content-Type: application/json
and would start to download it.
But this one would work for older IE and all new browsers
Content-Type: text/plain

When you send ajax request by jQuery just set header:
```JavaScript
$.ajax({
	url: ...,
	dataType: 'json',
	headers: {
		Accept: 'application/json'
	},
	accept: 'application/json'
}).done(function(data, textStatus, jqXHR) {
	...
});
```

and then server would respond with:
Content-Type: application/json

Also we should use
```php
JResponse::toString();
```
because in ajax method on server side there might be set some other headers by
```php
JResponse::setHeader(...);
```