fix SVG foreignObject (fix #4122)

src/core/vdom/create-element.js

@@ -55,8 +55,9 @@ function _createElement (
       // unknown or unlisted namespaced elements
       // check at runtime because it may get assigned a namespace when its
       // parent normalizes children
+      const childNs = tag === 'foreignObject' ? 'xhtml' : ns
       return new VNode(
-        tag, data, normalizeChildren(children, ns),
+        tag, data, normalizeChildren(children, childNs),
         undefined, undefined, ns, context
       )
     }

src/platforms/web/util/element.js

@@ -5,7 +5,8 @@ import { makeMap } from 'shared/util'
 
 export const namespaceMap = {
   svg: 'http://www.w3.org/2000/svg',
-  math: 'http://www.w3.org/1998/Math/MathML'
+  math: 'http://www.w3.org/1998/Math/MathML',
+  xhtml: 'http://www.w3.org/1999/xhtm'
 }
 
 export const isHTMLTag = makeMap(