Added picture to isHTMLTag map (#5675)

This change will make sure the below error is not seen while unit testing components using <picture> in their template.
[Vue warn]: Unknown custom element: <picture> - did you register the component correctly? For recursive components, make sure to provide the "name" option.

Author: timbenniks

Diff for: src/platforms/web/util/element.js

@@ -11,7 +11,7 @@ export const namespaceMap = {
 export const isHTMLTag = makeMap(
   'html,body,base,head,link,meta,style,title,' +
   'address,article,aside,footer,header,h1,h2,h3,h4,h5,h6,hgroup,nav,section,' +
-  'div,dd,dl,dt,figcaption,figure,hr,img,li,main,ol,p,pre,ul,' +
+  'div,dd,dl,dt,figcaption,figure,picture,hr,img,li,main,ol,p,pre,ul,' +
   'a,b,abbr,bdi,bdo,br,cite,code,data,dfn,em,i,kbd,mark,q,rp,rt,rtc,ruby,' +
   's,samp,small,span,strong,sub,sup,time,u,var,wbr,area,audio,map,track,video,' +
   'embed,object,param,source,canvas,script,noscript,del,ins,' +