Add fix for $.fn.data calls

When calling `$(el).data()`, the received object contains camel-cased keys, this fixes issue jquery#436

Author: liady

src/jquery/data.js

@@ -2,6 +2,7 @@ import { migrateWarn } from "../main.js";
 import { camelCase } from "../utils.js";
 
 var oldData = jQuery.data;
+var oldFnData = jQuery.fn.data;
 
 jQuery.data = function( elem, name, value ) {
 	var curData, sameKeys, key;
@@ -38,3 +39,37 @@ jQuery.data = function( elem, name, value ) {
 
 	return oldData.apply( this, arguments );
 };
+
+jQuery.fn.extend({
+    data: function(key, value) {
+        if (arguments.length === 0 && typeof Proxy !== 'undefined') {
+            var result = oldFnData.call(this);
+            return new Proxy(result, {
+                get: function(target, prop) {
+                    if (
+                        prop !== camelCase(prop) &&
+                        target[prop] === undefined
+                    ) {
+                        migrateWarn(
+                            'jQuery.data() always sets/gets camelCased names: ' +
+                                prop
+                        );
+                        return target[camelCase(prop)];
+                    }
+                    return target[prop];
+                }
+            });
+        }
+        if (arguments.length > 0 && typeof key === 'string' && key !== camelCase(key)) {
+            migrateWarn(
+                'jQuery.data() always sets/gets camelCased names: ' + key
+            );
+            var args =
+                arguments.length > 1
+                    ? [camelCase(key), value]
+                    : [camelCase(key)];
+            return oldFnData.apply(this, args);
+        }
+        return oldFnData.apply(this, arguments);
+    }
+});