fix($sanitize): dont not remove tab index property

Closes #8371
Closes #5853

Author: lgalfaso

src/ngSanitize/sanitize.js

@@ -228,7 +228,7 @@ var uriAttrs = makeMap("background,cite,href,longdesc,src,usemap,xlink:href");
 var htmlAttrs = makeMap('abbr,align,alt,axis,bgcolor,border,cellpadding,cellspacing,class,clear,' +
     'color,cols,colspan,compact,coords,dir,face,headers,height,hreflang,hspace,' +
     'ismap,lang,language,nohref,nowrap,rel,rev,rows,rowspan,rules,' +
-    'scope,scrolling,shape,size,span,start,summary,target,title,type,' +
+    'scope,scrolling,shape,size,span,start,summary,tabindex,target,title,type,' +
     'valign,value,vspace,width');
 
 // SVG attributes (without "id" and "name" attributes)