add object assign polyfill

MediaBrowser.WebDashboard/MediaBrowser.WebDashboard.csproj

@@ -269,6 +269,9 @@
     <Content Include="dashboard-ui\legacy\dashboard.js">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
+    <Content Include="dashboard-ui\legacy\objectassign.js">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
     <Content Include="dashboard-ui\livetvguideprovider.html">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>