Resources not always disposed

[jeroenheijmans]

In the RestClient.cs there is the following code:

csharp-rest-api/MessageBird/Net/RestClient.cs

Lines 203 to 206 in c6e2199

	using (var requestWriter = new StreamWriter(request.GetRequestStream()))
	{
	requestWriter.Write(body);
	}

Looking at the source for StreamWriter in .NET Core latest (should be similar in other runtimes) this only closes but does not dispose the inner stream.

It might be prudent to change the start of the code to:

using (var stream = request.GetRequestStream())
using (var requestWriter = new StreamWriter(stream))
{
    requestWriter.Write(body);
}

The specific reason I'm asking for this, is that we're seeing ErrorExceptions with inner SocketExceptions claiming AddressAlreadyInUse (10048) errors. This happens after we send a few thousand SMSs, and seems to indicate that sockets on the host are being consumed faster than they become available. So maybe if we don't wait for the GC to collect the .GetRequestStream(), and instead dispose via a using as I suggest, things might improve.