Use a different workaround for detecting when Opera finds a status 304 page. Fixes #6060.

src/ajax.js

@@ -629,9 +629,11 @@ jQuery.extend( jQuery.ajax, {
 		try {
 			// IE error sometimes returns 1223 when it should be 204 so treat it as success, see #1450
 			return !xhr.status && location.protocol === "file:" ||
-				// Opera returns 0 when status is 304
 				( xhr.status >= 200 && xhr.status < 300 ) ||
-				xhr.status === 304 || xhr.status === 1223 || xhr.status === 0;
+				xhr.status === 304 || xhr.status === 1223 ||
+				// Opera returns a status of 0 for redirects -
+				// We can detect this by the fact that Opera also doesn't return any headers
+				xhr.status === 0 && !xhr.getAllResponseHeaders();
 		} catch(e) {}
 
 		return false;
@@ -651,7 +653,7 @@ jQuery.extend( jQuery.ajax, {
 		}
 
 		// Opera returns 0 when status is 304
-		return xhr.status === 304 || xhr.status === 0;
+		return xhr.status === 304 || xhr.status === 0 && !xhr.getAllResponseHeaders();
 	},
 
 	httpData: function( xhr, type, s ) {

test/data/notmodified.php

+<?php header('HTTP/1.0 304 Not Modified'); exit; ?>

test/unit/ajax.js

@@ -68,6 +68,18 @@ test("jQuery.ajax() - error callbacks", function() {
 	});
 });
 
+test(".ajax() - 304", function() {
+	expect( 1 );
+	stop();
+
+	jQuery.ajax({
+		url: url("data/notmodified.php"),
+		success: function(){ ok(true, "304 ok"); },
+		error: function(){ ok(false, "304 not ok "); },
+		complete: function(xhr){ start(); }
+	});
+});
+
 test(".load()) - 404 error callbacks", function() {
 	expect( 6 );
 	stop();