Implement a `DisposeAsync` method for the `BrotliStream` class

[Taritsyn]

When using the current implementation of BrotliStream class in ASP.NET Core 3.0 Preview 7 web application you may receive the following error:

InvalidOperationException: Synchronous operations are disallowed.
Call WriteAsync or set AllowSynchronousIO to true instead.

To fix this error, need to implement in the BrotliStream class a DisposeAsync method (included in .NET Standard 2.1).