Fix memory leak around handling of Accepts header.

When adding an Accepts header to the request, this shouldn't be put into
DefaultParameters.  Repeated requests through the same RestClient instance
were adding the Accepts header every time, resulting in a large memory
leak because DefaultParameters was growing without bound.

Instead, only add the header for this particular request.

RestSharp/RestClient.Async.cs

@@ -73,10 +73,6 @@ private RestRequestAsyncHandle ExecuteAsync(IRestRequest request, Action<IRestRe
 			var http = HttpFactory.Create();
 			AuthenticateIfNeeded(this, request);
 
-			// add Accept header based on registered deserializers
-			var accepts = string.Join(", ", AcceptTypes.ToArray());
-			this.AddDefaultParameter("Accept", accepts, ParameterType.HttpHeader);
-
 			ConfigureHttp(request, http);
 
 			var asyncHandle = new RestRequestAsyncHandle();

RestSharp/RestClient.Sync.cs

@@ -62,10 +62,6 @@ private IRestResponse Execute(IRestRequest request, string httpMethod, Func<IHtt
 		{
 			AuthenticateIfNeeded(this, request);
 
-			// add Accept header based on registered deserializers
-			var accepts = string.Join(", ", AcceptTypes.ToArray());
-			this.AddDefaultParameter("Accept", accepts, ParameterType.HttpHeader);
-
 			IRestResponse response = new RestResponse();
 			try
 			{

RestSharp/RestClient.cs

@@ -293,6 +293,13 @@ private void ConfigureHttp(IRestRequest request, IHttp http)
 				request.AddParameter(p);
 			}
 
+			// Add Accept header based on registered deserializers if none has been set by the caller.
+			if (!request.Parameters.Any(p2 => p2.Name.ToLowerInvariant() == "accept"))
+			{
+				var accepts = string.Join(", ", AcceptTypes.ToArray());
+				request.AddParameter("Accept", accepts, ParameterType.HttpHeader);
+			}
+
 			http.Url = BuildUri(request);
 
 			var userAgent = UserAgent ?? http.UserAgent;