Fixes #6230. Added a unit test to control that, since the ajax rewrit…

…e, setting the Accept header actually replaced the previous value and didn't append to it (tested in Safari 4.0.4 for which the problem was specifically reported).

test/unit/ajax.js

@@ -316,6 +316,28 @@ test(".ajax() - headers" , function() {
 
 });
 
+test(".ajax() - Accept header" , function() {
+
+	expect( 1 );
+
+	stop();
+
+	jQuery.ajax(url("data/headers.php?keys=accept"), {
+		headers: {
+			Accept: "very wrong accept value"
+		},
+		beforeSend: function( xhr ) {
+			xhr.setRequestHeader( "Accept", "*/*" );
+		},
+		success: function( data ) {
+			strictEqual( data , "accept: */*\n" , "Test Accept header is set to last value provided" );
+			start();
+		},
+		error: function(){ ok(false, "error"); }
+	});
+
+});
+
 test(".ajax() - contentType" , function() {
 
 	expect( 2 );