Mixpanel name_tag should prefer using email over user ID

analytics.js

@@ -1921,4 +1921,4 @@ analytics.addProvider('Vero', {
         window._veroq.push(['track', event, properties]);
     }
 
-});
+});

src/providers/mixpanel/mixpanel-test.js

@@ -114,7 +114,7 @@
 
         spy.reset();
         analytics.identify(userId, traits);
-        expect(spy.calledWith(userId)).to.be(true);
+        expect(spy.calledWith(traits.email)).to.be(true);
 
         spy.restore();
     });
@@ -176,4 +176,4 @@
         spy.restore();
     });
 
-}());
+}());

src/providers/mixpanel/mixpanel.js

@@ -87,7 +87,7 @@ analytics.addProvider('Mixpanel', {
         // against settings to make sure they're enabled.
         if (userId) {
             window.mixpanel.identify(userId);
-            if (this.settings.nameTag) window.mixpanel.name_tag(userId);
+            if (this.settings.nameTag) window.mixpanel.name_tag(traits && traits.$email || userId);
         }
         if (traits) {
             window.mixpanel.register(traits);