New XmlHttpRequest framework uses different metric for success

[crschmidt]

In Ajax.Request, we had:

return !status (status >=200 && status < 300);

as a metric for a 'successful' request.

Now, we have:

request.status >= 200 && request.status < 300

Tim's response to this was 'Don't depend on the framework to declare success'. If that's going to be the answer, that's fine: We should drop success/failure calls from the framework, and change all the OpenLayers code that uses them to instead do its own parsing of success/failure. The other option (in my opinion) is to make the success metric the same as it used to be -- which is less code, and fixes what is a regression from the current library to the previous library (in my opinion).

[tschaub]

allow for no status on requests

[tschaub]

Passes Firefox 2 and IE 7.

[tschaub]

(In [7609]) The Request functions now consider a request with null status as a sucessful request. This is relevant for the file 'protocol' where xhr status is never set. r=crschmidt (closes #1638)