Consider dropping GraphQL via GET for Mutations

[michaelstaib]

graphql/graphql-over-http#123

[michaelstaib]

We have reworked the middleware options and defaults.

By default we allow HTTP GET requests but only with query operations.

This can be changed with the following middleware options:

endpoints
    .MapGraphQL()
    .WithOptions(
        new GraphQLServerOptions 
        {
            AllowedGetOperations = AllowedGetOperations.QueryAndMutation
        });

In order to disable HTTP GET requests:

endpoints
    .MapGraphQL()
    .WithOptions(
        new GraphQLServerOptions 
        {
            EnableGetRequest = false
        });