Explicitly setting content header to avoid issues with some GraphQL servers. (#595)

* Explicitly setting content header to avoid issues with some GraphQL servers. For instance synthetic GraphQL generated out of Azure APIM.

Author: Vivek4Int

src/GraphQL.Client/GraphQLHttpRequest.cs

@@ -35,7 +35,10 @@ public virtual HttpRequestMessage ToHttpRequestMessage(GraphQLHttpClientOptions
         };
         message.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/graphql-response+json"));
         message.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
-        message.Headers.AcceptCharset.Add(new StringWithQualityHeaderValue("utf-8"));
+        message.Headers.AcceptCharset.Add(new StringWithQualityHeaderValue("utf-8"));
+
+        // Explicitly setting content header to avoid issues with some GrahQL servers
+        message.Content.Headers.ContentType = new MediaTypeHeaderValue(options.MediaType);
 
         if (options.DefaultUserAgentRequestHeader != null)
             message.Headers.UserAgent.Add(options.DefaultUserAgentRequestHeader);