Override the default ACCEPT header value

I'm trying to use the GraphQL.Client library in a project that interacts with a 3rd party API. Currently, the API only accepts `application/json` for the ACCEPT header. Is there an easy way to override the library's default value of `application/graphql-response+json, application/json` (https://github.com/graphql-dotnet/graphql-client/blob/master/src/GraphQL.Client/GraphQLHttpRequest.cs#L45)? I tried the following code, but it did not work.

```
using var graphQLClient = new GraphQLHttpClient("https://3rdpartyservice.com/graphql", new NewtonsoftJsonSerializer());
graphQLClient.HttpClient.DefaultRequestHeaders.Accept.Clear();
graphQLClient.HttpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
```

I understand the library was updated to follow the spec for GraphQL over HTTP (https://github.com/graphql-dotnet/graphql-client/pull/459), but that is still in the draft phase, and it appears the company that maintains the API hasn't implemented the spec's requirement. I've opened a ticket with the company to see if they can update their API to more closely conform to the draft spec, In the meantime, I wanted to see if there was an easy way to get my project working.