When executing Async requests on multiple threads with 1 restclient, there is sometimes an exception

[nickvane]

When configuring a RestClient and using that client to execute multiple threaded executes, this works most of the time, but sometimes gives following exception:

System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
at System.Collections.Generic.List`1.Enumerator.MoveNext()
at RestSharp.RestClient.ConfigureHttp(IRestRequest request, IHttp http)
at RestSharp.RestClient.ExecuteAsync(IRestRequest request, Action`2 callback, String httpMethod, Func`4 getWebRequest)
at RestSharp.RestClient.ExecuteAsync(IRestRequest request, Action`2 callback)
at RestSharp.RestClient.ExecuteAsync[T](IRestRequest request, Action`2 callback)
at RestSharp.RestClientExtensions.ExecuteAsync[T](IRestClient client, IRestRequest request, Action`1 callback)

This happens because the defaultparameter collection is changed on each execute.
https://github.com/restsharp/RestSharp/blob/master/RestSharp/RestClient.Async.cs line 78

// add Accept header based on registered deserializers
var accepts = string.Join(", ", AcceptTypes.ToArray());
this.AddDefaultParameter("Accept", accepts, ParameterType.HttpHeader);

ConfigureHttp(request, http);

The ConfigureHttp method in https://github.com/restsharp/RestSharp/blob/master/RestSharp/RestClient.cs line 286

// move RestClient.DefaultParameters into Request.Parameters
foreach(var p in DefaultParameters)
{
    if(request.Parameters.Any(p2 => p2.Name == p.Name && p2.Type == p.Type))
    {
        continue;
    }
    request.AddParameter(p);
}

enumerates over the collection which could be changed while enumerating because an execute on another thread adds a parameter to the default parameter collection of the restclient.

I think that adding the accept headers in each request is not necessary. On the contrary, for each new request a parameter is added to the collection and this makes the collection constantly grow.
I would suggest moving the adding of the accept header out of the Execute method, although I don't immediately see another method where this would fit.