Servlet exception mapper cleanup

[Vadym S]

Spark servlet exception mapper is not cleaned after server stopped. I've created a test to illustrate issue we are facing: master...sukolenvo:test-servlet-clean

Here you can see I am starting server foo, stopping it, and starting another server bar. Although server foo was stopped, error handler is still returning foo.

I suggest to add something like ExceptionMapper.getServletInstance().clear() to SparkFilter.destory()

Can open PR for test and/or fix if needed.