Bug 986544 - Don't try to parse searchplugins list as XML. r=rnewman

XHR requests default to text/xml.  This just avoids trying to interpret
list.txt as XML.

toolkit/components/search/nsSearchService.js

@@ -3674,6 +3674,7 @@ SearchService.prototype = {
         let deferred = Promise.defer();
         let request = Cc["@mozilla.org/xmlextras/xmlhttprequest;1"].
                         createInstance(Ci.nsIXMLHttpRequest);
+        request.overrideMimeType("text/plain");
         request.onload = function(aEvent) {
           deferred.resolve(aEvent.target.responseText);
         };