Fixes #13815: Ensure each element has its own private data object - Tests by @rwldrn

src/data.js

@@ -90,13 +90,9 @@ Data.prototype = {
 
 		// Handle: [ owner, { properties } ] args
 		} else {
-			// Support an expectation from the old data system where plain
-			// objects used to initialize would be set to the cache by
-			// reference, instead of having properties and values copied.
-			// Note, this will kill the connection between
-			// "this.cache[ unlock ]" and "cache"
+			// Fresh assignments by object are shallow copied
 			if ( jQuery.isEmptyObject( cache ) ) {
-				this.cache[ unlock ] = data;
+				jQuery.extend( this.cache[ unlock ], data );
 			// Otherwise, copy the properties one-by-one to the cache object
 			} else {
 				for ( prop in data ) {
@@ -104,6 +100,7 @@ Data.prototype = {
 				}
 			}
 		}
+		return cache;
 	},
 	get: function( owner, key ) {
 		// Either a valid cache is found, or will be created.