Adds Func<Stream, Task> to replace Action<Stream> for GetObjectAsync()

[ebozduman]

Fixes #420

Adds Func<Stream, Task> to replace Action to run GetObjectAsync with async callback.

This also fixes the disposed object exception

Unhandled exception. System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'SslStream'.
   at System.Net.Security.SslStream.<ThrowIfExceptional>g__ThrowExceptional|138_0(ExceptionDispatchInfo e)
   at System.Net.Security.SslStream.ReadAsync(Memory`1 buffer, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnection.FillAsync(Boolean async)
   at System.Net.Http.HttpConnection.CopyToContentLengthAsync(Stream destination, Boolean async, UInt64 length, Int32 bufferSize, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnection.ContentLengthReadStream.CompleteCopyToAsync(Task copyTask, CancellationToken cancellationToken)
   at Minio.MinioClient.<>c__DisplayClass111_0.<<getObjectFileAsync>b__0>d.MoveNext() in /home/ersan/work/src/github.com/minio/minio-dotnet/Minio/Helper/OperationsHelper.cs:line 112
--- End of stack trace from previous location ---
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_1(Object state)
   at System.Threading.QueueUserWorkItemCallbackDefaultContext.Execute()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart()
   at System.Threading.Thread.StartCallback()