Added an ability to add headers before writing GraphQL response.

[donbeave]

Hello!

This PR adds a small but quite useful feature, it modifies the original GraphQLServletListener by adding an additional method void beforeFlush(HttpServletRequest request, HttpServletResponse response), which will be called right before the response will be written and flushed.

In our use-case it can be used to add headers like API rate limit or some temporal tokens generated after some GraphQL mutations were called together.