fix target _blank when anchor has children (#1290)

* fix target _blank when anchor has children

Couple of days ago, we have faced an issue with out non-SPA project:
Once you add a child element to a ``<router-link>`` with ``target="_blank"`` attribute, it wouldn't fire on a new tab. check this fiddle:
https://jsfiddle.net/asynchronous/dnLs1dm4/10/

I've changed ``gaurdEvent`` method in ``link`` component so it could fix this issue.

* Update link.js

* Update link.js

Author: RezaHaidari

src/components/link.js

@@ -91,8 +91,8 @@ function guardEvent (e) {
   // don't redirect on right click
   if (e.button !== undefined && e.button !== 0) return
   // don't redirect if `target="_blank"`
-  if (e.target && e.target.getAttribute) {
-    const target = e.target.getAttribute('target')
+  if (e.currentTarget && e.currentTarget.getAttribute) {
+    const target = e.currentTarget.getAttribute('target')
     if (/\b_blank\b/i.test(target)) return
   }
   // this may be a Weex event which doesn't have this method