Cross origin AJAX request always preflighted

[Arjen]

By setting a custom header, X-Requested-With, all cross-domain ajax request will be pre-flighted and need an extra OPTIONS http request.

​http://www.w3.org/TR/cors/#cross-origin-request

"If the force preflight flag is false and the following conditions are all true, follow the simple cross-origin request algorithm:

• The request method is a simple method.
• Each of the custom request headers is a simple header or custom request headers is empty."

So GET, HEAD and POST calls with 'simple' headers (​http://www.w3.org/TR/cors/#simple-header) don't need preflight.

The are some restrictions on the allowed values for the Content-Type header, only application/x-www-form-urlencoded, multipart/form-data, or text/plain are considered 'safe' (​https://developer.mozilla.org/en/HTTP_access_control#Preflighted_requests) This is also mentioned in de WD: ​http://www.w3.org/TR/cors/#design-decision-faq

Suggested fix:

1. Don't set a custom X-Requested-With header at all. They don't provide any extra security at all.
2. Only set the header when request must be preflighted, i.e. has custom headers, type is not one of GET, POST or HEAD or Content-Type is not 'safe'.

[jaubourg]

Fixes #8423. Never set X-Requested-With header automagically for cross-domain requests.

Changeset: 6c124d3dd47fb399c7512c5c3b3420e438c32b65