[Urss] mime type fixes

[Mike Tillberg]

   I was having trouble viewing certain feeds in Moz1.5/Linux, and it
seems to have been caused by the mime type of the document.  I made a
few small changes to the RSSLoader that seems to have fixed things.

Before myXMLHTTPRequest.open(), I added
myXMLHTTPRequest.overrideMimeType("text/xml") to force an appropriate
mime type.

Second, responseXML still wasn't being populated correctly, so I replaced:

if(this.myXMLHTTPRequest.responseXML)

with

this.rssDoc = this.myXMLHTTPRequest.responseXML;
if (this.rssDoc == null) {	
    this.rssDoc = (new DOMParser())
	.parseFromString(new String(this.myXMLHTTPRequest.responseText),
			 "text/xml");
}
if(this.rssDoc)