Don't show open/save dialog in addition to style installation prompt

This apparently has been happening for a year, since #617.

Might be better to show open/save on cancel, as implemented for the
RIS/BibTeX prompt in 4ecdd55 (which caused this), but that would have to
wait for the better promise handling in 5.0. Doing "Save Link As..." for
a CSL file also seems a bit more straightforward than for RIS/BibTeX,
which can be served in complicated ways.

chrome/content/zotero/xpcom/mimeTypeHandler.js

@@ -345,7 +345,7 @@ Zotero.MIMETypeHandler = new function () {
 			Zotero.debug(e);
 		}
 		
-		if (!handled) {
+		if (handled === false) {
 			// handle using nsIExternalHelperAppService
 			var externalHelperAppService = Components.classes["@mozilla.org/uriloader/external-helper-app-service;1"].
 				getService(Components.interfaces.nsIExternalHelperAppService);