Add test for request decompression

[Shane32]

I considered adding a section to the readme as follows:

Request decompression

ASP.NET Core supports request decompression independently of GraphQL. Please see the Microsoft documentation for details on how to configure your application for request decompression. Browsers do not normally compress requests, so this is typically only useful for non-browser clients.

However, I decided that it was such a rare need that it was not worth adding to the readme.