Update ResourceHandler.cs

Added override for passing in the file ext to correctly set the correct mime type
APPVEYOR_REPO_TAG sniffing for $version
Try to appease AV
Added null checks for managedCefBrowserAdapter.
Merge pull request #635 from dhasi/wpf_cwb_nullRef

Added null checks for managedCefBrowserAdapter.
Merge pull request #629 from Jayman1305/patch-2

Update ResourceHandler.cs
ResourceHandler formatting change (indentation)
Merge pull request #634 from jornh/feature/av-nuget-tags

Have appveyor set nuget dependency versions based on tags
Merge remote-tracking branch 'remotes/upstream/master' into cef/2171