Fixes #5812. =? will be detected even when it has been escaped during data serialization.

src/ajax/jsonp.js

 (function( jQuery ) {
 
 var jsc = jQuery.now(),
-	jsre = /\=\?(&|$)/,
+	jsre = /\=(?:\?|%3F)(&|$)/i,
 	rquery_jsonp = /\?/;
 
 // Default jsonp settings

test/unit/ajax.js

@@ -1081,7 +1081,7 @@ test("jQuery.getScript(String, Function) - no callback", function() {
 });
 
 test("jQuery.ajax() - JSONP, Local", function() {
-	expect(9);
+	expect(10);
 
 	var count = 0;
 	function plus(){ if ( ++count == 9 ) start(); }
@@ -1128,6 +1128,22 @@ test("jQuery.ajax() - JSONP, Local", function() {
 		}
 	});
 
+	jQuery.ajax({
+		url: "data/jsonp.php",
+		dataType: "jsonp",
+		data: {
+			callback: "?"
+		},
+		success: function(data){
+			ok( data.data, "JSON results returned (GET, processed data callback)" );
+			plus();
+		},
+		error: function(data){
+			ok( false, "Ajax error JSON (GET, processed data callback)" );
+			plus();
+		}
+	});
+
 	jQuery.ajax({
 		url: "data/jsonp.php",
 		dataType: "jsonp",