Better support for file:// urls

[ahocevar]

In OpenLayers.Request, we check for the status code when we decide whether to trigger the "success" event or not. In both Firefox and Safari, AJAX requests to  file:// resources work fine, but this status check keeps them from being processed properly, because the status code for such requests, even when successful, is 0.

This ticket suggests to set the status code to 200 when we encounter a  file:// url and have a responseText.

[ahocevar]

Tests still pass. Thanks for any review.

[bartvde]

Andreas, this looks good to me.

jQuery took a similar approach:  https://github.com/jquery/jquery/commit/cb85da7b62e74e7339ed652ade209aaffbd0c15b/

[ahocevar]

(In [11711]) better support for  file:// resources. r=bartvde (closes #3174)