Responses to POST can be cached for future GET methods

[darrelmiller]

https://github.com/aspnet/ResponseCaching/blob/chrross/initial/src/Microsoft.AspNet.ResponseCaching/CachingContext.cs#L39

This is a weird one that requires using GET as the cache key instead of using the original request method.

[aspnet-hello]

This issue was moved to dotnet/aspnetcore#2628