add an async version of the WriteZipOutputStream benchmark

Author: Numpsy

.gitignore

@@ -253,3 +253,4 @@ paket-files/
 /test/ICSharpCode.SharpZipLib.TestBootstrapper/Properties/launchSettings.json
 _testRunner/
 docs/help/api/.manifest
+/benchmark/ICSharpCode.SharpZipLib.Benchmark/BenchmarkDotNet.Artifacts/results

benchmark/ICSharpCode.SharpZipLib.Benchmark/Zip/ZipOutputStream.cs

@@ -1,5 +1,6 @@
 using System;
 using System.IO;
+using System.Threading.Tasks;
 using BenchmarkDotNet.Attributes;
 
 namespace ICSharpCode.SharpZipLib.Benchmark.Zip
@@ -37,5 +38,25 @@ public long WriteZipOutputStream()
 				return memoryStream.Position;
 			}
 		}
+
+		[Benchmark]
+		public async Task<long> WriteZipOutputStreamAsync()
+		{
+			using (var memoryStream = new MemoryStream(outputBuffer))
+			{
+				using (var zipOutputStream = new SharpZipLib.Zip.ZipOutputStream(memoryStream))
+				{
+					zipOutputStream.IsStreamOwner = false;
+					zipOutputStream.PutNextEntry(new SharpZipLib.Zip.ZipEntry("0"));
+
+					for (int i = 0; i < ChunkCount; i++)
+					{
+						await zipOutputStream.WriteAsync(inputBuffer, 0, inputBuffer.Length);
+					}
+				}
+
+				return memoryStream.Position;
+			}
+		}
 	}
 }