The existing web.config should not be overridden and deleted

[mark-cordell]

With --webserver=path option, if there is an existing web.config file in the parent directory, the default web.config file that is written can conflict with it, and cause the server to return a 500 error. I can work around this by creating my own web.config in the acme-challenge directory, and I can test this by dropping a file with no extension in the directory and verify that I can browse to it and get a 200 response with the correct content-type. However, the current implementation overwrites my working web.config with a non working one.